To millions of movie fans, stars are synonymous with the roles that made them famous. Think Humphrey Bogart in “Casablanca” or Harrison Ford as Indiana Jones.

While many struggling actors dream of getting such a leading role, there is a double responsibility with that. The leading man has to carry the film: If the music, art direction or a supporting performance is off, the film can still work. But the leading role has to keep the audience focused and engaged. Yet a leading actor’s duties start from the first day of lensing, helping to set the tone during filming, particularly for fellow actors. That duty can weigh heavily for veteran stars. It’s especially daunting — and rewarding — for actors who are relatively new to that role, including this year’s Jeremy Renner in “The Hurt Locker,” Paul Bettany in “Creation” and Ben Foster in “The Messenger.” And for a comparative veteran like Matt Damon, those duties can offer a new challenge when given an image-bending role, as in “The Informant.”

Foster’s army sergeant carries nearly every scene in Oren Moverman’s war-at-home drama “The Messenger.”

“Traditionally, we like our leads to be accessible,” says the actor, whose eclectic credits range from “Freaks and Geeks” to “3:10 to Yuma.” “The lead is the journey you take as an audience member. If you’re not willing to take that emotional ride with a particular actor, then it’s very difficult for the film to succeed.”

As Foster notes, a movie might present elements that flow together flawlessly — aided by camerawork, sound design and editing — but if the perf falls short, the film could crumble. “If the centerpiece does not support the spokes on the wheel,” Foster analogizes, “the wheel will fall apart.”

The responsibility on John Amiel’s biopic of Charles Darwin, “Creation,” was “daunting,” says Bettany.

“There’s a lot written about (Darwin) which is a double-edged sword,” he states. “There are piles and piles of books and so much research available, and yet if you’re the sort of human being I am, you’re always a bit nervous that perhaps you are doing the role a tremendous injustice.”

But where there’s pressure, there’s also opportunity to help instill a sense of camaraderie. “The tone on set is determined in part by the leading actor,” Bettany declares. “This tone determines if it’s a creative environment or a competitive environment or a dead frightening environment. It’s hard to be creative when you’re frightened, and I believe you can achieve far more if you are sharing the scene with somebody and trying to become more than just your separate parts. That’s when your role as the lead feels useful and creative.”

For Renner, star of Kathryn Bigelow’s “Hurt Locker,” the payoff comes not from having the most screen time or dialogue. The actor’s main role is to focus on character and how he fits into the bigger picture. “‘The Hurt Locker’ is not story-driven but character-driven, so its success depends heavily on the actor,” Renner says. “But being a lead or not, I like to fight to make each film the best film it can be. I want to help make it great, to make it interesting and to tell a story as truthfully as possible. That’s our job as actors — to elevate the material as best as we possibly can and turning it into reality.”

Foster concludes, “As the lead, as cold and chilly as that term sometimes sounds, you have room to take your time with this character’s experience and experience it for yourself. It’s about how you connect to the world, about how we listen to each other. The opportunity to ask questions about this character in depth with people you want to ask them with is a wonderful challenge. It really is a gift.”