Some movies boast casts of thousands. “Locke” relies on just one actor, a virtually unrecognizable Tom Hardy, who calmly juggles multiple crises via cellphone from behind the wheel of his BMW SUV.

These days, the character’s situation might be a metaphor for the film’s writer-director, Steven Knight. With “Locke” having premiered Jan. 17 in Sundance, and a raft of scripts in various stages of production or development — including projects for Helen Mirren (the Oprah Winfrey-produced “The Hundred-Foot Journey”), Tobey Maguire (Bobby Fischer biopic “Pawn Sacrifice”) and Jeff Bridges (big-budget fantasy “Seventh Son”) — Knight could hardly be busier.

Along with “Locke,” he directed a second feature last year: Jason Statham starrer “Redemption,” a sincere, yet staggeringly unconventional romance between a soup-kitchen nun and a homeless Afghanistan war veteran — “a story I didn’t think other directors would go for,” he says. Audiences didn’t go for it either. The film grossed a mere $37,000 domestically.

Knight is drawn to characters in the shadows, people whom other writers consider too marginal to explore. “If you’re looking at an illegal minicab, the story of the driver almost always will be more interesting than the story of the passenger,” he says of the script that earned him an Oscar nomination, 2002’s “Dirty Pretty Things.”

And then there’s “Locke,” a logistical high-wire act in which Knight positioned three Red cameras around the exterior of a moving BMW and filmed Hardy’s character as he attempts to deal with a major event in his private life at the same time he handles a defining moment at work.

“I told Tom, ‘I want Ivan Locke to be the most ordinary person in the world. Nothing about him is exceptional. He’s got two kids, he’s got the dullest job,’” recalls Knight, who cast Hardy to a play an even-tempered construction manager having a spectacularly challenging day.

In retrospect, Knight concedes that he may have been a bit overconfident about the transition to directing features, despite having helmed several episodes of “The Detectives” for BBC and watching such helmers as Stephen Frears, Michael Apted and David Cronenberg interpret his scripts.

When it came time to try his hand at directing, however, Knight discovered that the nature of reality — freezing 4 a.m. winter shoots in Soho for “Redemption,” rigorous 26-minute segments along the highway for “Locke” — prevented him from translating the precisely imagined films in his head to the screen.

“As a writer, working is warm and dry, and you choose your hours,” he says. “But when you start doing the actual process (of directing a movie), it really is like a bucking bronco.”

Still, for all his commercial success as a screenwriter, Knight was beginning to feel boxed in by a medium he suspected was capable of much more.

“Film is governed by very strict rules, more severe than visual art or theater,” he says. “And yet, I defy anyone to say life has three acts. Those restrictions get imposed for a reason, which is because people need to get their money back. If you want to tell a story where the hero’s not the hero, the heroine’s not the heroine, and they do wrong things, you have to go out and do it yourself. That’s where ‘Locke’ came from.”

Knight had the idea of trying to stage a dramatic, theater-style experience in an unconventional venue: a moving vehicle — an introspective space that, thanks to cellphones and modern technology, still provides accessibility to a person’s most important contacts.

“What I wanted to do was make a very ordinary tragedy, he explains. “When you’re watching lots of cars going past, you can imagine that within those

bubbles of light, similar tragedies are taking place.”

Working on a modest budget, Knight created an environment in which to test such cinematic conventions. Having done that has given him the confidence to keep challenging rules that others treat as if they come with the camera itself.

Next up, he says he plans to direct another experimental performance piece. “Hopefully with Tom,” he adds.