Raising a question
When I ask the students in my film classes at the university if they can define or at least somehow describe what makes a film a documentary, they usually come up with answers like this: "It is a type of film that is based on the real world and real people, depicting things as they are or telling about historical events in a supposedly truthful or objective manner." Or they say that it has to do with a certain realism of style and that it is "filming on some real location without actors, artificial props or a pre-constructed narrative." Sometimes they also just cite the title of a classic book on the subject saying that it is "representing reality."  And often enough, Danish students go on to talk a lot about "facts" and "truth" as a necessary condition for non-fiction film. Some smart guy may even suggest that it is nothing but the opposite of fiction.
If they tend to agree too much or too early on this (and I have nothing else prepared for a three-hour lecture), I can usually revitalize the discussion by asking if my cousin can be justified in claiming that he is working on documentary films, when in fact what he does for a living is to install surveillance cameras at gas-stations and supermarkets. After all, this does seem to meet the criteria of representing reality, of filming without the use of actors, and recording as truthfully as possible what is actually there - and it is not fiction.
At this point, some students will begin to argue that certainly this mechanical type of recording and displaying video does not make the man a documentary filmmaker; we need to see some artistic point of view, a message of some sort, a moral or ideological ambition with the film made - in short, a wish to make a difference, to change the world, or at least the way in which some relevant audience will look upon the world or themselves. A documentary film director may adopt the so-called "observational" mode of filming or try to be like "a fly on the wall" - but this is a process demanding a lot of choices both in the recording and in the editing phase. It is not just about recording what is there; it is also about selecting and presenting and editing in such a way that we see present conditions as wrong and begin to look for alternatives that should be brought about. Documentary film- making - and also the reception of documentary films - is all about ethics, politics and an aesthetic approach, and as such it is a highly subjective or personal matter, it is now argued.
To counter this I can ask whether we could not say the same about most fiction films: are they not all very personal, do they not have some sort of moral or even ideological viewpoint built into their characters, action and location, and are they not intended to qualify as artistic and aesthetic products making the world a little better and the audience a bit more enlightened about human life?
Definitions and definitions
At this point I have usually caused a lot of confusion in the classroom - and in my own mind as well - about the nature of documentary films. Can the concept be defined, or is it just some slippery term that we happen to apply in many different ways. "And so what?" a student may ask. Well, I still think it is important, at least as an academic exercise, to try to pin down what we mean by "documentary." And not just in academic discussions about film theory does it make a difference how we conceive of a major genre or label a specific film: in everyday life we navigate through the schedules for TV-programs or film festivals using terms like fiction, documentary, drama, reportage, comedy, and nature film. We seem to know quite well and instantaneously what a documentary is and would probably call it ridiculous and feel cheated if someone labeled the recent Disney production Pirates of the Caribbean - Dead Man's Chest a documentary. But why so? After all there were pirates in the Caribbean once, were there not?
Definitions can be of many sorts, depending on how strict we want to be. A proper definition (or a definition of essence) would characterize exactly what it is that makes up this group of films, and would spell it out in such a clear way that it would be easy to assess whether a specific film belonged or did not belong to this group or genre. What are the necessary and sufficient features? How does this type of film differ from other types of film?
Reality, representation and presentation
The example above with the surveillance camera indicates that "recording reality" is too vague a criterion, and not just because "reality" sooner or later becomes a very difficult concept to narrow down (just think about "reality-TV" programs in which almost everything is a construction). The continuous mechanical recording of a raw tape lacks the touch of someone selecting and editing for the purpose of expressing or communicating something to someone. Both fiction and non-fiction films differ markedly from a simple mirroring or duplicative function. This is among other things revealed through the camerawork, i.e. all the intentional changes such as camera moves, cuts, composition, all sorts of adjustments that come from human intervention, and through the post-production process of organizing various sound tracks and visual tracks into a whole that was not there before. Time may be condensed and the chronology changed, music, subtitles, or voice-over added, shots may be interlaced or interrupted by wipes, etc. As a rule of thumb, a film is hardly a film without camera work, cuts or editing, and it is neither a fiction film nor a documentary if it is nothing more than a "re-presentation" of what happened to be in front of a lens and a microphone.
A film is not a mere representation, but a willed presentation of something made by someone in a specific way and for someone. The phrase "representation of reality" is utterly mistaken as a definition of documentary, because the idea of film as mirroring is a false one and a very misleading ideal. Also the term "reality" is confusing: it may have the straightforward positive connotations of facing reality and seeing things as they really are, but often enough it is interpreted by students in theoretical discussions as just filming "normally" in an "objective" way without being creative or manipulative. Just the facts…. But trying to make "a correspondence with actual facts" and "objective and neutral reproduction" the core characteristics of documentary is naïve in the sense that it has the same weaknesses as philosophical positivism. To believe that reality is made up first by objective facts and secondly by subjective or personal sentiment is to make you yourself blind and deaf to the prevailing power structures and ideologies of this world. "Let us stick to the facts and not be subjective and emotional" - that is the anxious mantra of those not wanting or daring to work for any change or a proper overview and perspective on things.
Truth and creativity
This however does not mean that it is all right to disregard facts or to tell a lie in a non-fiction film. But it must be noted that the "truth" of a film can be understood in other ways. A lot of facts or statements about facts that can be verified may be present even in a fiction film. The whole story may be pure fantasy, the characters fictitious and the behavior of the actors may consist of incredible stunts - but still the film may be striving for "truth" in another sense of the word: true emotions and perhaps even to illustrate some more general truths about human life.
Lacking a good definition of its essence, it could be an idea to look at the etymology and history of the term. The word documentary has its root in the Latin word "docere" which meant to teach or instruct. We also know the more modern and common phrase that something is "a document" (e.g. an important piece of paper presented in court) and we may ask someone "to document" his identity or statements. Within film history, the term seems to have been used first by John Grierson who wrote about Robert Flaherty's film Moana (1926) that it had "documentary value." And indeed it can be a nice academic exercise to go through the records of how different film critics and writers or even film directors themselves have used the term.
John Grierson, known as the founder of the classic British documentary movement in the 1930's, coined the phrase "creative treatment of actuality." It often comes as a surprise to my students today to see that even at that time, the creativity of the documentary enterprise was underlined. They generally assume that that is a modern invention. Also I can usually surprise them with the highly poetic and almost rap-like ending of the classic film Night Mail (1936) as well as with the meta-filmic approach of Dziga Vertov's Man With a Movie Camera (1929).
Working on a clear picture - a positive approach
But after all this confusion I owe it to my students to come up with a more positive approach as to how to define documentary films. What I suggest then, is not a proper definition of essence, nor a canon or list of traditionally accepted masterpieces, nor do I give up completely and say that you may call anything a documentary. What I offer is a list of points to consider, almost like a doctor's list of symptoms to be checked before prescribing a certain medicine. We do have in our trained minds a certain general picture or idea of what the term "documentary" means or how it is used by our friends or scholars, but in applying it to a specific film we have to make an individual assessment, looking at the pros and cons. The specific film should meet most of the criteria on the list, but it is hard to say how many or which is the most important.
No single criterion seems to qualify or disqualify a given film. For example, it is often considered that actors belong to fiction films and not to a true documentary (unless, of course, they are portraying themselves). On the other hand there are exceptions that we are ready to accept, such as a TV-documentary using professional actors to re-enact a crime scene in order to make us understand how something may have happened. Indeed it would be immoral to have the real criminal perform another knife-stabbing on the real victim - even though that could be said to be more true or closer to the original event.
My list of points to consider
So here are some of the features to be considered before accepting the label "documentary." I'll start the list with some points that underline the great variety of the genre. Some of the different modes may even be seen as partial descriptions of subgenres:
Functions of the film, metaphorically described (by personification): 
A documentary film can be seen to function as a prophet - explorer - painter - advocate - bugler - prosecutor - observer - catalyst - guerrilla - performer - therapist - spin doctor. (I once misspelled "bugler" as "burglar," but perhaps that might be another possibility for the filmmaker.)
Possible modes or narrative strategies:
Expository: lecturing, overtly didactic, e.g. with a personal presenter or an explanatory voice-over.
Observational: like a "fly on the wall," the camera, microphone and film crew seem not to be disturbing the scene or even to be noticed by the participants.
Participatory or interactive: the film crew takes part in the action or chain of events.
Reflexive: the film exposes and discusses its own role as a film (e.g. the ethics or conditions of filmmaking) alongside the treatment of the case or subject.
Performative: the film crew creates many of the events and situations to be filmed by their own intervention or through events carried out for the sake of the film.
Poetic: the aesthetic aspects, the qualities of the form and the sensual appeals are predominant.
Ways of being true. Documentaries seem to have a certain obligation towards "truth." This may be understood, however, in different ways:
Correspondence: statements and details of film are not lies or fiction but in accordance with actual or historical facts, events and persons.
Coherence: the film constitutes a well-argued, non-contradictory whole.
Pragmatic or conventionalist view: the film is in line with predominant views and general, long termed discursive practice.
Relativism or constructivism: as you like, or how we make sense of things.
Illumination theory of truth: to become enlightened, to see and hear and understand more, to become inspired and gain insight (perhaps recollection).
More points to consider:
Intentions of the filmmaker: enthusiasm and commitment, the filmmaker wants to explore, to probe and to show us something important or otherwise overlooked; devoted to a cause or to people, trying to make a difference (not just making money, having fun or exposing herself).
Subject matter, themes or content: something of importance and relevance; historical, social or natural phenomena; persons and places of significance. (Note, however, that modern TV-audiences seem to find significance in what critics may call rather trivial "everyday documentaries" (in Danish "hverdagsdokumentar.")
Expectations of the (general) audience: authenticity, insight, disclosure, something about real people and problems, learning something.
Target groups (implied): general public (public service), or segments with a more specialized interest and knowledge on the subject in question.
Ethics: we expect truthfulness, not lies or distortion, even when the film is committed to high ideals and values. Propaganda is over the line (difficult to define too, my provocative suggestion is: "propaganda is a documentary made by my enemy"). The documentary may be engaged and enthusiastic, but should be open about its preferences, sympathies and presuppositions. "Neutrality" or "objectivity" should be understood as problematic, but a well-balanced view is welcomed. The film may reflect its own intervening and perhaps ethically problematic role in relation to participants and general context. Carefulness, but also boldness in addressing tabooed subjects.
Communicative function: to inform, discuss, engage, enlighten, intervene, explore, express, disturb and commit - more so than to merely entertain, amuse, distract, conform or confirm (e.g. a religious or political community).
Labeling: sponsors, critics, distributors, professionals, scholars, curators, librarians, editors of TV- and film-programs would characterize this as a documentary.
Popular, lay opinion, everyday language: films received and talked about in accordance with the tradition, similar to other so-called documentaries or non-fiction films.
Context of actual use: education, public service (as image or part of an obligation for the distributor), debate forum, campaigns, discussions and pastime entertainment (e.g. in the cabin on an airplane flight).
Style and form: often realism, perhaps with a reportage-like style, interviews, a rough style, lighting and settings and sound appear natural and not carefully controlled (contrary to smooth and slick lighting, camera movements, montage and continuity of classic Hollywood style). Often an argumentative, exploring or investigative attitude, often thematic more than dramatic.
Relation to major genres and art: it is not fiction, it can be seen as belonging to one of the main genres of rhetoric: judicial, epideictic or political. It may be highly artistic and poetic, but seems more like art with a purpose than art for art's own sake. Epics, lyrics and drama seem to serve the didactic aspect.
Recordings: on location, authentic settings and props, real time, real sound, no actors or acting, but actual people (or animals, in nature documentaries) being themselves. Drama and narrative appear not imposed on the scenes, but emerging from the actual (pro-filmic) events.
Editing: the rhetorical structure appears to be more important than ordinary dramatic continuity; the rate of manipulation and rearrangement of picture and sound seems low. A voice-over commentary or text-streaming is more likely than extensive use of non-diegetic music. The mixing of heterogeneous material (e.g. recordings from a different time or location) is accounted for.
Context of viewing or distribution: e.g. the Discovery Channel, educational TV, TV-slots or festivals announced as documentary, educational institutions, films shown within organizations and companies.
Importance and evaluation: In terms of context and communicative qualities, the film makes a considerable contribution towards a better world…
To be continued
With the last entry here about the importance of a film in a larger context, I may be crossing the line between describing documentary and prescribing what I think it should be. But actually I believe this is in harmony with the ambition of both past and present documentary productions - that is, those of a certain quality, of course.
I am well aware that this is not a systematic list and that several points could or should be improved upon or added by others.  Probably this list is also subject to change not only as we become wiser, but as the history of the genre develops further. But whether it is the content or the label that is subject to change when we consider the historical development of documentary film - well, I'll have to ask my students about that…
1 Bill Nichols, Representing Reality (Indiana University Press, 1991).
2 Inspired by Eric Barnow in Documentary - a history of the non-fiction film (Oxford University Press, 1974).
3 Based on Bill Nichols' work, e.g. Introduction to Documentary (Indiana University Press, 2001).
4 Some suggestions and tips for further study: