The 33-year-old actor broke into features in 1993 with a minor role in the comedy-drama, "There Goes My Baby," starring Dermot Mulroney and Rick Schroder. He appeared in a succession of low-budget and independent features, eventually playing the male lead in the 1997 TV movie, "On the 2nd Day of Christmas," co-starring with Mary Stuart Masterson.
In 1998, Ruffalo shared the screen with other emerging stars including Steve Zahn, Sam Rockwell and Paul Giamatti in "Safe Men" -- the first feature for writer-director John Hamburg, who went on to co-write the screenplays for the Ben Stiller hits, "Meet the Parents" and "Zoolander."
Following appearances in "54" and Ang Lee's "Ride With the Devil," Ruffalo hit the jackpot with "You Can Count on Me," as Terry Prescott -- the wayward brother who re-enters the life of his single-mom sister after being out of touch for months. Laura Linney earned an Oscar nomination for her portrayal as the sister, Sammy, and writer-director Kenneth Lonergan was nominated for an Oscar for best original screenplay.
Ruffalo's career took off.
"The Last Castle" director Rod Lurie said that as soon as he saw Ruffalo in "You Can Count on Me," he knew the actor would be perfect to play Yates, a young soldier doing time in a military prison where Redford leads an inmate insurrection against the warden, played by James Gandolfini, the star of the HBO hit, "The Sopranos."
After the first scene between Redford and Gandolfini, there is no doubt which one is going to be on the right side of the argument the movie makes about the difference between following orders and doing the honorable thing.
Yates, however, is a different story.
The prison bookie, he will take action on anything from basketball games to how long a new inmate will survive the brutal conditions at "the castle," as the military prison is known. He seems to have an aversion to loyalties of any kind, and his is one of the few characters who provide any suspense about which side he will take in the final battle between the inmates and the order-obsessed warden.
Ruffalo said that's an interesting assignment for an actor.
"It's a juggling act," he said. "When you finally do choose sides, it has to be believable."
Ruffalo said the challenge with a role like that is to see "how much you can commit to playing on the fence and not be fearful of people thinking you're a bad guy."
A believer in the complexity of human nature, Ruffalo said he saw the character engaged in a struggle between what he knows is right and his own broken spirit as a convicted prisoner.
In fact, Ruffalo said the central idea of "The Last Castle" is that the human spirit is, finally, unbreakable.
"There's a certain pride in these prisoners," he said. "They're still ostensibly soldiers, trained to have honor and respect. They obviously committed themselves to the loyalty of our country at some point."
Ruffalo said the movie celebrates the "really important things about being an American" -- individualism, the love of freedom and justice.
"Redford comes and he sees conditions are subhuman and he wants to change it, and he calls upon the men to bring out the best in themselves," he said.
Before Sept. 11 -- when terrorists destroyed the World Trade Center Twin Towers and inflicted heavy damage on the Pentagon, using hijacked airliners as bombs -- DreamWorks' promotion of the movie featured an upside-down American flag -- an official sign of distress.
After the terrorist attacks, the studio changed the artwork, and Ruffalo -- among others -- wondered how the movie would be received in the marketplace.
"I was a little nervous about it at one point," he said. "It is men actually turning on part of the government. But that is what makes America great, is that we can do that. That's how we basically ended up being a free country.
"We said(to England), no, this is substandard and we made our own country. That's what these men do, they rebel against something that's dehumanizing."
Still, Ruffalo said "The Last Castle" is going to be "very provocative."
The movie, Redford's first feature since "The Horse Whisperer" in 1998, contains more than its share of crowd-pleasing moments, owing largely to the decision to set up the bad guy early and watch him absorb a series of blows along the way. The warden, Col. Winter, is a close relative of James Cagney's Captain Morton in "Mister Roberts" and Humphrey Bogart's Lieutenant Commander Philip Francis Queeg in "The Caine Mutiny."
Redford vs. Gandolfini may also remind viewers of the power struggle between Jack Nicholson's R.P. McMurphy and Louise Fletcher's Nurse Mildred Ratched in "One Flew Over the Cuckoo's Nest" -- particularly in the way it is resolved.
Ruffalo said the production, filmed almost entirely on location at the historic Tennessee State Penitentiary, proceeded without the normal compliment of studio "suits," because it was shot at a time when Hollywood studios were cranking out as much product as they could in anticipation of possible strikes last year by writers and actors. He said DreamWorks had four features in production simultaneously.
"They couldn't be as hands-on as they would usually be," he said, "so we were kind of left on our own a lot of the time. We'd see certain pieces of the script and say this isn't working, and change as we'd go along -- which you never would do."
Kind of like being a garage band?
"Yes," said Ruffalo, "but a multi-million dollar garage band."