No-one likes a prescriptivist, but I’m going to put on that hat for a moment anyway. It is preferable, I think, that movies should be movies first and foremost - that whatever it is they’re doing, it’s something that couldn’t be done in any other medium or art form. And this, I think further, is not a minimal requirement met by Locke, a fairly terrific exercise in writing psychologically revelatory dialogue to be delivered by a solitary actor giving everything he has to digging into that psychology. Except that as exercises go, this one is particularly exercisey never quite managing to make the character it depicts and his shifting perspective more important than the show-off hook that underpins the whole game. And even if it did, I don’t honestly know that Locke the one-man show in a black box theater would be even slightly less effective than Locke the one-man movie in a single tightly confined set.

Of course, there’s always the argument that Locke the movie is responsible for capturing the Tom Hardy performance without which it would surely not be so effective as it is, and for that, I am glad that the film exists as a film. Even if Hardy’s performance, however spectacularly difficult it must have been to do the heavy lifting in a film where his face and only his face is the focal point of something like 95% of the shots, is not without its limitations. Certainly, it’s not the all-time career peak some have noticed (I’d take his work in Bronson any day of the week), and if nothing else, he’d need to have some points taken away for the inexplicable accent he’s elected to sport for the 85-minute duration of the film, an effortful attempt at Welsh by way of Austria. Compounding things is that there’s no obvious reasons for the character to be Welsh in the first place: the name Ivan Locke certainly doesn’t suggest that, and when we hear his children speak on the phone, they sound nothing like him.

It’s a bizarre, off-putting misstep in a film that, relying completely on the strength of its central performance, can’t afford a single one, but if you can get over that hump (which took me fully 15 minutes), Hardy’s performance is an impressive display of carefully choosing a subtle range of expressions and weighted lines to give a perfectly-modulated performance of what amounts to a monologue for the intimacy of the movie camera. It’s quintessentially the kind of acting that works only if you fluently speak the language in which it was delivered, and can catch the interplay of vocal cadence with the movements of his face (since you are reading this English-language review of the English-language film, I take it this won’t be a problem), and like the film containing it, it always feels just a little bit like an experiment for the sake of it. But it’s a terribly impressive display of what film acting in 2013/2014 is capable of from a purely technical standpoint even so.

Ivan Locke, for the record, is driving a care late at night. For the first 10 or 15 minutes, we get to play at being detectives while he makes a number of organically cryptic phone calls to his family explaining that he has a work crisis and his work explaining that he has a family crisis. Both of which are true, as it turns out, though a bit of sleight-of-hand is involved. For he does indeed have a problem with his job as a construction manager, finding that the enormous delivery of cement he had planned for the morning has been hobbled by political and practical mistakes, and his second-in-command is by no means ready to deal with those issues. And he does indeed have a family problem, though it has nothing to do with his wife and sons, but with the baby about to be born as a result of an ill-advised affair with a middle-aged colleague. She's gone into premature labor, and things aren't looking great; Locke's current jog down the M1 is taking him to by with her out of a sense of grim duty. And as we learn during the downtimes when Locke isn't busily calling this person or that trying to keep his life from splitting off in a million directions, that sense of duty is directly related to his feelings of resentment to his deceased father, an alcoholic who abandoned his son in childhood.

Setting aside issues of taste (I think the movie arrives at the reveal that Locke is having a bastard child, by a woman he has no deep affection for and regrets sleeping with, far too early), this is all pretty crafty writing exploring a man's difficult battle with his demons, and other than the comedy sketch Welsh accent, Hardy does sterling work in evoking the shifting balance of self-loathing, pride in doing the right thing by everybody (and his astonishment that his wife might not be willing to play by his script for how they're going to rebuild), and the fatigued but iron-willed behavior of a professional who really doesn't want to deal with this shit right now, but the shit must be dealt with regardless. As an exercise in pure character building, it's a great one.

As an exercise in cinema, though, it's a little whiffy. Steven Knight writes a hell of a screenplay, giving the voice-only characters on the phone just enough distinct personality that it doesn't feel like we're trapped inside one man's head, while simultaneously making the exploration of that same man ring with authenticity and subtle touches. But his directing isn't up to the same level: he and cinematographer Haris Zambarloukos come up with a pretty limited repertoire of shots from various angles inside Locke's car, and the switch from one image to the next speaks less to editor Justine Wright's desire (or the flexibility of the footage she was given, more likely) to create a pulsing rhythm or using the visuals to punctuate the emotional beats of the story, than to the filmmakers' hope that if they cut often enough, the audience won't get bored. This ain't Buried, the last big "confined space on a phone" movie, where the camera was used to shape and intensify the space. It's just a film that takes place in a car, shot with little versatility or poetry. Obviously, that's not the be-all and end-all of art: the writing and acting are the reasons Locke exists, and they are just things. But they're not enough to help the film make the jump from a worthy experiment if you're in the mood for such things, to a great piece of cinema that's absolutely worth seeing.