If you had to identify a specific period when the modern documentary was born, it would have to be the 1960s and a movement that was known in the UK as Free Cinema, in the US as Direct Cinema, and in France as Cinéma Vérité. Made possible by the arrival of more portable film cameras and (a little later) crystal synchronised tape recorders, the movement threw off the formal, scripted and tripod-based formula of contemporary documentary works in favour of hand-held camerawork and a free-form approach to the subject matter. Direct Cinema filmmakers would select their subject and then follow them with the camera, intruding as little as possible, shooting reel after reel of footage and creating the story in the editing room. Revolutionary at the time, its influence is still seen today in works as diverse as Super Size Me and character-led TV docusoaps like BBC's Airport. Direct Cinema films had no commentary, little in the way of on-screen captions or graphics, no formal interviews, and a sometimes intimate relationship with their subjects. These films got us close to the people they featured in a way that was largely new to the documentary medium, catching them in offhand moments, in states of discomfort and distress as well as triumph. The so-called Reality TV of today, the docusoaps, the fly-on-the-wall documentaries we have become all too familiar with all owe their existence to the pioneers of this movement.

A key early work of the American Direct Cinema movement was Robert Drew's Primary (1960), which followed a young political hopeful named John F. Kennedy during his campaign for the 1960 Wisconsin primary of the title. A ground-breaking work in many respects, its crew included many of the future stars of the Direct Cinema movement, including cameramen Richard Leacock (who went on to co-direct A Stravinski Portrait in 1966), D.A. Pennebaker (one of the leading Direct Cinema directors, whose work includes the landmark 1966 Bob Dylan documentary Dont Look Back and the 1968 Montery Pop) and Albert Maysles, who together with his brother David were to become one of the most influential and celebrated of Direct Cinema filmmaking teams. For a number of years the Maysles brothers crewed for other directors and made their own short films, including A Visit With Truman Capote and Meet Marlon Brando (both 1966), but it was in 1969 when they really made their mark on the documentary scene with Salesman.

The film follows the fortunes of four Irish-American family bible salesmen – Paul Brennan, Charles McDevitt, James Baker and Raymond Martos – all of whom have adopted animal nicknames: The Badger, The Gipper, The Rabbit and The Bull respectively. Though all are tellingly introduced in the first five minutes of the film, the focus of attention soon becomes Paul, The Badger, a seasoned salesman who appears to be losing his touch.

It's hard to imagine the impact this film must have had on its release, but even today it makes for compelling cinema. Compiled from over 100 hours of footage by editor Charlotte Zwerin and co-director David Maysles, what unfolds is a film that fully delivers on a variety of levels, as a fascinating study of an occupation that has since been consigned to history, as a portrait of a man on a downward career slide, as a sly comment on the exploitation of the poor, and as a critique of capitalism and religion, two of the defining aspects of modern America that here have become inextricably entwined.

Those familiar with David Mamet's play (or James Foley's splendid film adaptation) Glengarry Glen Ross (1992) will find themselves immediately at home here, from the varying personalities and persuasive techniques of the salesmen to the bullish pep talks and the dead-end leads. There are even times when Paul comes across as a real life Shelley Levine – his best sales years behind him, he gripes endlessly at the impossibility of making a sale in districts that present few problems for his colleagues, and seems increasingly weary of the whole salesman lifestyle. Initially it's quite hard to warm to Paul, a man of Irish descent who misses no opportunity to ridicule his own people and who is openly contemptuous of other ethnic communities ("No more fucking Italian food for me after these guineas," he bleats after another fruitless day). But as the film progresses it becomes clear these are symptoms of a man who is past his professional prime and for whom every day is a combination of defeat and humiliation, something emphasised when Charlie takes him out on a sale and makes a point of bluntly outlining his failings to a customer.

This, it becomes clear, is no way for anyone with a conscience to make a living. Knowing full well that their banter will cut no ice in wealthy districts, the salesmen specifically target the poorer communities, selling to people who live from paycheque to paycheque a product that none of them need and few really want. These men live on commission and the sale is everything, and what happens afterwards is just not their concern. This is most starkly illustrated when Paul lives up to his nickname by badgering a woman for money under the false pretext that he is the area manager following up on a sale that her husband has already agreed to, even though she clearly does not want the book and would rather confer with her husband before handing over the money, something Paul clearly has no intention of letting her do. Elsewhere we watch two of the salesman relentlessly push their wares to a woman who struggles to find a way to convince them that she simply cannot afford to buy what they are selling. As the camera closes in on her face, highlighting her anxiety, I wanted to run into the room and scream, "Leave the poor woman alone! She can't afford it! She doesn't NEED it!"

Their sales technique preys on the deeply held religious beliefs of their potential customers, but is pushing the word of God as a product on a par with a vacuum cleaner or aluminium (sorry, aluminum) siding. Religion has become just another marketable commodity, part of the economic food chain in which those who have money feed off of those who do not. To the families whose homes the salesmen wheedle their way into, religion is an integral part of their life, but to the salesmen and their bosses it's a new car or a new house – it's social status and the material trappings of success.

Stripped of many of the components of modern documentary works – there is no presenter, no voice-over, no fancy graphics, no postmodernist emphasis on technique and presentation – Salesman seems as fresh now as it ever did. The virtual invisibility of its filmmaking hides a very precisely constructed narrative whose storytelling subtlety and identification with character aligns it to some of the finer works of modern European, Middle Eastern and Soviet cinema, and the purity and immediacy of its approach very clearly link the principals of Direct Cinema to those of the later Danish-led Dogme 95 movement.

Salesman is, above all, a fascinating and ultimately sobering character study, a microcosm of materialism at its most superfluous in which those without the money to spend are effectively conned into buying a product on the false promise of salvation. It is a testament to the filmmakers' skill that even as we despair at what Paul does for a living, his slow deterioration and disillusion with his profession ultimately leave us saddened, not for the salesman, but for the man.

Shot on 16mm Plus-X with a speed push to enable the Maysles to continue shooting when the light level drops, the picture displays a very visible level of grain but a generally very nice tonal range. The emphasis on grays rather than deep blacks can't help but feel appropriate to the subject matter, but the black levels are solid enough when they need to be. A single bounced light was occasionally used in darker interiors, but otherwise the film was shot by available light, with all of the issues that this can bring. Those expecting the crispness of modern film and digital works might be disappointed, but anyone used to tape copies of Direct Cinema and Vérité films will be more than happy with Criterion's transfer. Some scratches and dirt are evident, but are not distracting. The picture is framed in its original 1.33:1 aspect ratio.

The original mono track is included here and does display some of the problems caused by working on a low budget with a directional, hand-held mic, especially when dealing with thick Boston accents in sound-unfriendly locations. Dialogue occasionally slips into indistinct mumblings, and for the untrained ear even more clearly spoken sentences can prompt a rewind or activation of the subtitles for confirmation of what was said. Some scenes also display a noticeable shift in voice quality from shot to shot, but this reflects sound recording issues of the time and it is to Criterion's credit that we are presented with the soundtrack as it was originally heard and that it has not been remixed or re-jigged. For the most part the dialogue is clear enough, and the soundtrack is very clean and free of pops and noise.

Acknowledging the importance and influential nature of the film, Criterion have provided a very nice range of extras to accompany it.

Probably the best, and a real treat for documentary fans, is a commentary track by co-director Albert Maysles and editor Charlotte Zwerin. In typical Criterion style, the two have been recorded separately and edited together to form a virtually continuous track, and even putting aside its historical value, this is a compelling and wonderfully informative commentary. The two outline the genesis of the film and its sociopolitical intentions, provide a lovely level of technical detail about the production and information on the filming of specific scenes, and fill us in on what happened after filming was complete. It was good to hear that the Maysles became friends with Paul, but to have Albert point out in one early shot signs of the arthritis that eventually killed him just adds to the pathos of the character. Charlotte enforces the importance of the editing to the success of the film, something she feels too often passes without comment, though Albert describes her as "a genius," also citing her work on Gimme Shelter and her own projects as director as further evidence. This is great stuff for all potential documentary filmmakers and film enthusiasts alike.

Every bit as valuable is an Interview With Albert and David Maysles (31:17), which was recorded for the WCBS TV series Camera Three and first aired in 1969, the year of the film's release. The interview is divided into eight titled chapters and conducted by Newsweek critic Jack Kroll, shot 4:3 on video and in colour. Despite minor fuzziness and smearing, it's in pretty good shape. To put faces to the names and voices is pleasing enough, but the brothers cover a fair amount of interesting ground, from how they approached the process of filming unannounced in people's houses to their dislike of the term 'Cinéma Vérité', an opinion they share with fellow Direct Cinema filmmaker Frederick Wiseman. They sometimes disagree with their interviewer and even with each other, but are very clear about the thinking behind the film and the expected audience reaction, making this is a useful extra, it being a record of their views at the time of the film's release rather than a retrospective one.

The Rabbit on NPR's 'Weekend Edition' (11:27) is a short radio interview with James Baker, aka The Rabbit, recorded in January 2000. Baker looks back at how he first became a bible salesman and the techniques he used to get invited into people's homes. His son Jimmy also chips in towards the end. One thing – don't get interrupted when listening to this, as I did, twice – you can't pause, rewind or fast-forward it, so I had to start it again each time.

Photo album is a gallery of production photos divided into two sections: The Salesmen consists of 50 promotional stills from the film, many of which are actual film grabs, while Behind-the-Scenes With the Filmmakers has 45 stills made up of production photos of the Maysles at work, opening night press photos and poster artwork. All are black-and-white and reproduced close to full screen.

Theatrical trailer (3:14) gives a fair introduction to the style and content of the film and even rounds off in modern style with a collection of positive press quotes.

Filmographies are provided for the Maysles Brothers and Charlette Zwerin.

Salesman is a key work of the Direct Cinema movement, a compelling and ultimately saddening study of a man in decline in an industry that may now hock energy suppliers and double glazing instead of family bibles, but whose techniques of persuasion remain depressingly familiar. There's no better introduction to the work of the Maysles Brothers, who are still quoted today as a major influence by directors such as Joe Berlinger and Bruce Sinofsky (the Paradise Lost films), and Criterion's disc once again does the film proud. If you are at all interested in the history of the documentary form, or perhaps studying it at college or university, then this disc has to be considered an essential purchase.