Like all "Single-Set" films, the potential for showiness and self-aggrandizement is extreme. And when I first read the details (and early reviews) of Locke's journey, I wondered if Hardy was actor enough to make it feel like an actual story rather than a cinematic exercise.
Happily, I can report that I stopped thinking about the film's central "conceit" almost immediately. Yes, we were watching a single actor in a single car, and nothing more. But that singularity, rather than growing dull (as one might expect) morphed almost instantly into "Can't Tear My Eyes Away" material.
Two things of particular note: First, the film makes nearly no effort to set the stage, strapping us into the car beside Locke and sending us off with very little backstory; mere hints, really, which we brush up against from time to time during the course of his conversations. Sure, we can piece the past together bit by bit, but the film's not interested in the past (except for insofar as it shapes and motivates its protagonist). Instead, it's a snapshot of a single (tumultuous) span of time in the life of a single man -- a decent enough chap, it seems, but one who finds himself forced to confront the devastation wrought by his past transgressions.
Second, Hardy's performance was much, much quieter and far subtler than I was expecting. And that's good, because a scenery-chewing, dominant approach -- of which the charismatic Hardy is more than capable -- would have pretty much destroyed the smallness of the film. In fact, there were moments when I wondered if the script could have benefited from a bit more of that same subtlety; it felt a trifle on-the-nose as the end drew near. That's a fairly small criticism, though, in an otherwise-impressive example of undertaking and overcoming an incredibly-specific (and often-abused) cinematic challenge without allowing the formal requirements to overwhelm or obscure the actual story being told.
Be warned; adult themes and fairly-prolific language render this a film for more "mature" audiences. If you're up for that sort of thing, though, you will not be disappointed.