While Jude Law often plays a dreamy romantic lead onscreen, the English actor successfully stepped out of his comfort zone for the upcoming film Black Sea.

Director Kevin Macdonald admitted he was initially skeptical about giving Law the lead role in his submarine thriller, but changed his mind once he witnessed the actor's passion.

"You've seen them do the same sorts of things for years and then to think they could actually do something completely different takes a moment," Macdonald told us during a New York City roundtable. "He convinced me just by being passionate about it and dedicating a huge part of his life for several months to it."

Indeed, Macdonald made the right choice casting Law as Captain Robinson, a character whose obsession with money severely clouds his judgment by allowing him to make decisions that could potentially kill himself and the crew. Law became totally immersed in the character by transforming his physique (he got buff), training with the Navy in the Mediterranean and putting on a very specific Scottish accent, one that Macdonald said is not easy to pull off.

The suspenseful film - written by Dennis Kelly - follows Captain Robinson and his Russian/British crew members as they take a dangerous trip into the depths of the Black Sea to find a sunken Nazi boat rumored to be holding tons of treasure. The movie explores ethnic conflict, greed, the growing wealth gap and the fear of the unknown, while leaving viewers wondering about the crew's fate.

"Psychologically, [submarine movies] are very interesting. There's often a focus on the decay of the hierarchy of the crew," he said. "That kind of thing of somebody going a little bit crazy, and I guess that's all to do with the fact that you are in this confined, claustrophobic environment, and that does happen on submarines. It does happen that people go bonkers."

Macdonald, who directed 2012's Marley and 2006's The Last King of Scotland, explained the difficulties that come with filming in that claustrophobic state. For the first two weeks, Macdonald filmed the cast on an actual vintage Russian submarine just outside of London.

"In the real submarine you can't move the walls, you can't move things out of the way. You're very pressed up against the wall the whole time," he said. "When we went on to the set, which we were on after that, I think all the actors... remembered what it was like being on the real sub and so that definitely changed their performance."

Black Sea, which opens in theaters on January 23, showcases one terrifying situation after another, as the crew attempts to survive the unknown with tensions mounting.

"[It's] like being in space," Macdonald said about being underwater. "You shouldn't be there. You're only alive because of the machine that you are living in and the terror of that is what initially attracted me to it."

Tags: Movies