Mark Ruffalo, star of the just-released Sundance winner You Can Count on Me, may not be a household name yet, but that's not to say moviegoers don't know him.

"People say that I remind them of their little brother a lot," Ruffalo tells TV Guide Online. "[Or they] think that I look like [an] evil Donny Osmond."

Well, if You Can Count on Me becomes the hit some predict, Osmond soon may be referred to as the good Mark Ruffalo. The film — which is generating Oscar buzz for Ruffalo and co-star Laura Linney — explores the relationship between a brother and sister orphaned at a young age.

"I liked how complex he was," Ruffalo says of his character, an irresponsible yet well-intentioned drifter. "He's got a lot of different emotional notes: He's angry but he's also really open; he's frustrated but [also] willing to hear; he's cruel but, at the same time, very caring. As an actor, it's really exciting [and] challenging to play those different kinds of beats in the same person."

Although Ruffalo has appeared in the indies Committed, Ride with the Devil and 54, he's better known for his stage work in such Off Broadway plays as This is Our Youth and Avenue A. In fact, he'll likely reunite with You Can Count on Me's writer/director Kenneth Lonergan this spring for the play Lobby Hero.

"I definitely want to do films, but I like the idea of balancing it out with theater," says the actor, who currently is shooting John Woo's big-budget flick, Wind Talkers. "There's something [challenging] about really having your ass on the line every night and having to fill that world completely and straight through with no retakes."

Ruffalo is considerably less enthusiastic about another stress-inducing aspect of acting: Having critics compare you to Hollywood legend Marlon Brando. "I think he's a great American treasure," sighs the actor, "[but] to be put up against him is really setting me up for a fall. It puts a lot of pressure on me."

"[Besides], I want to have my own thing," he adds. "I want people to go, 'Oh, he's Mark Ruffalo.'"