Tom Hardy isn't supposed to be this good an actor. He's that guy built like The Hulk who bashes people in the face, right? He's the metal-masked villain Bane in "The Dark Knight Rises" and the brutal Virginia moonshiner Forrest Bondurant in "Lawless." He's the destructive Pittsburgh cage fighter Tommy Conlon in "Warrior" and "Britain's most violent prisoner," bloody and naked much of the time, in "Bronson."

But now here he is in "Locke," driving a BMW in a yuppie sweater, delivering one of the most subtle and powerful film performances of the year. "Locke" is unlikely to be wildly commercial: It consists solely of one man driving in a car, engaged in speakerphone calls, for 90 minutes. Ivan Locke is a construction manager who builds foundations with concrete, but the solid world he has built for himself has started to crack and tremble. He has potentially career-ruining business problems at work to resolve. He has problems at home that really can't be solved. He's driving away from something and toward something, determined to do the right thing, despite the consequences he will endure. Speaking in a soothing Welsh accent borrowed from Richard Burton, he strives to remain steady while the voices coming into his car panic. The movie premiered to raves at the Venice Film Festival in September and opens in U.S. theaters on April 25.

"If you're going to have one actor on screen for that length of time, you have to have the best," says Steven Knight, who wrote and directed "Locke." "And in my opinion Tom is the best actor we have. We being Europe."

Wait—Batman's nemesis is the best actor in Europe? And there's something else it's easy to get wrong about Mr. Hardy.

"I'm not huge at all," says the London-born Mr. Hardy. "I've had lots of lads tell me 'I thought you were bigger.'" He's 5-foot-9. His normal weight is about 158 pounds, and even for shirtless, muscle-bound roles in "Warrior" and "Bronson" he only beefed up into the 180s. "There's ways of creating an image—camera angles, intention, voice. You put the weight on in places where the camera's going to pick it up. You stand in a way that the camera's going to pick up. I would move as if my limbs were much heavier, move a bit like a boat on the water."