“All my life I’ve been alone…I’m God’s lonely man…”

The words belong to Travis Bickle, scrawled in the Taxi Driver‘s private journal, but how easily could they also be a half-conscious thought let slip by freelance surveillance expert Harry Caul during another night’s restive sleep, isolated at the heart of The Conversation.

One of the key movies of the 1970s, Francis Ford Coppola directed The Conversation (1974) between The Godfather (1972) and The Godfather Part II (1974). Compared with – and perhaps as a reaction to – the sweep of those films, it is an infinitely small work, but also intensively concentrated, burrowing inside itself until there seems no way back out. Rarely has a film been so completely about the stuff of its protagonist’s life.

Substituting the aural for the visual, The Conversation is Coppola’s reworking of Michelangelo Antonioni’s existential murder mystery, Blow-Up (1966), the film in which fashion photographer David Hemmings hunted the grainy terrain of a single photograph for evidence of a killing, while London froze swinging around him. But where Antonioni’s film has been somewhat sabotaged by its half-cocked attempts at freezing the Sixties moment, The Conversation, in its chill modernism and in the unforced ways it captures the drift of a city, remains utterly contemporary.

Gene Hackman plays Caul, in virtually a solo turn – much like the solitary saxophone his character plays in his monk’s cell of an apartment at night. At first, the name sounds like a shout, ironically chiming with his own calling, as someone who captures sound for a living. Eventually, though, Hackman spells it out for us: C-A-U-L, as in the protective membrane enveloping a foetus in the womb, echoed by the flimsy grey raincoat he wraps constantly around himself.

Painfully reserved, Hackman is just superb here. One year later, he would play another obsessive Harry with a need to discover, in Arthur Penn’s equally alienated Night Moves, and Harry Caul similarly resounds through Hackman’s canny appearance in Tony Scott’s action-heavy updating of the surveillance genre, Enemy of the State, a kind of unspoken, pumped up sequel to Coppola’s movie.

The first thing that catches attention in this film about sound is the figure of a mime (another Blow Up nod), the most animated of the tiny human dots moving around the city square over which the camera hovers at the opening. It’s lunchtime, close to Christmas, and the golden winter sunlight has brought the people out from the surrounding buildings.

The next thing we notice are anonymous men on roofs and hanging out of windows, aiming microphones that look like snipers’ guns. We begin to hear the electronically distorted sounds they are picking up, tracking a conversation between a young couple in the midst of the throng.

These men with  microphones work for Caul, who has been employed to record this specific, seemingly banal conversation – some idle chat about Christmas presents and drunks – for a faceless executive (later revealed as an unbilled Robert Duvall) who sits enshrined within a monumentally oppressive office block.

Famed throughout the surveillance community as the best in the business of finding things out, the assiduously private Caul remains unknown to the few people coming anywhere near him, drawing a careful veil around his inner life and his past, to such an extent that in all likelihood there are no secrets there at all, just more empty spaces. His right-hand man, Stan (another shining performance in the too-brief career of John Cazale), frustrated at being fed only need-to-know scraps, eventually quits him for another employer, one he can have an occasional beer with, and ends up working for Caul’s biggest rival, further fuelling Caul’s mounting paranoia.

Coppola gradually cuts Hackman off from his surroundings, isolating him behind partially opaque shields: curtains, plastic screens, glass walls. As he works at putting the taped conversation together piece by piece (Coppola’s camera lingering over the technology, Hackman’s fingers caressing the buttons and dials of tape machines, hypnotically raising and lowering levels, gradually filtering out extraneous noise), Caul deduces the young couple are having an affair, and he begins to suspect that his client, the woman’s husband, means to kill them, a situation which rekindles his lingering guilt over a previous job he carried out, which led to a series of murders.

The film that ushered Watergate into the cinema, The Conversation boasts one of the greatest of all movie endings, leaving Caul in a hopelessly desolate place, abandoned in his own worst nightmare. However, it’s just about possible to register a hint of acceptance as he finally settles into his fate. A devout Roman Catholic, the knowledge that he lives his entire life under constant supervision is already part of Caul’s lonely deal with God. He’s already committed to a lifetime playing his sad saxophone for unseen ears. This is one of the performances to show in any argument about whether Gene Hackman is one of the most important actors American cinema has ever had. It might also be Coppola’s finest movie. Give or take one or two.