"He never looked liked me. His physiognomy is not the same, and I'm not going to do a Dana Carvey. That's of no use in a drama, in a piece of fiction. There's no reality to it," Cromwell said in an interview.
A veteran of radical politics and activism since the 1960s, the 68-year-old Cromwell also found nothing to identify with in the policies of the two Bush administrations.
So Cromwell relied on his personal experience as a son and father to burrow into the character.
"It's a relationship between a father and son based on basically my experience with my father, who was actually of Bush's father's generation," Cromwell said. "And my relationship with my sons, all the 400 blows that always exist between father and son, parent and child. Unperceived, unacknowledged often on the one hand, and deeply felt and resented on the other side."
"W." presents the younger Bush (Josh Brolin) in earlier years as the black sheep of a family dynasty, a man continually disappointing his imperious father with his boozing and business failures.
At one point, Cromwell's Bush and Brolin's W. nearly come to blows in the family living room. Even after the son becomes president himself, he has a bizarre dream of being taunted for his inadequacies by his dad in the Oval Office.
An Academy Award nominee as the kindly Farmer Hoggett in "Babe," Cromwell has built a career as one of Hollywood's most-respected character actors. He had a memorable role as Queen Elizabeth II's husband in 2006's "The Queen," and has appeared in "Spider-Man 3," "L.A. Confidential," "The Green Mile" and "I, Robot."
Some top Hollywood stars turned down the role as the elder Bush, but Stone said he was happy to cast Cromwell.
"The key is the movie doesn't have anybody that's too famous. I think it helps the movie that there's a series of very solid character actors, and nobody overweighs the role. Nobody overbalances the movie," Stone said. "Jamie always was a wonderful character actor of major proportions. I was lucky I got turned down by more famous people."
Cromwell said he hoped "W." might prompt viewers to put more thought into their votes for president in November. But amid the war on terror and the economic crisis, the nation's future hinges more on the actions of average Americans than decisions from the White House, he said.
"The responsibility is ours. I think we're going to learn that," Cromwell said. "By ours, I mean the citizens. If this country is going to survive, it's going to survive on the backs of Americans, not because of a president."