Focus : Acting Presidential : GARY SINISE PLAYS HARRY S. TRUMAN TO THE LAST DETAIL IN HBO FILM

Gary Sinise immersed himself so deeply in his latest role as President Harry S. Truman that at one point director Frank Pierson got a little confused.

About a third of the way through the making of “Truman,” airing Saturday on HBO, Pierson recalls, he found himself “leaning over to Gary to give him some notes on a scene we had just done. We were going to reshoot it. I heard myself saying, ‘ Mr. President, would you mind doing that line this way?”’

Pierson wasn’t the only one convinced that Sinise was the 33rd President. Though Sinise never got to meet Truman’s daughter Margaret during production this spring, he was introduced to one of her sons on the set. “He called me ‘Grandpa,’ ” Sinise says with a smile. “G-R-A-N-D-P-A.”’

“Truman,” based on David McCullough’s bestseller, marks the first time the 39-year-old Sinise has played the central character in a film. The award-winning actor and director, who founded Chicago’s acclaimed Steppenwolf Theatre Company, signed on to play Truman soon after receiving an Oscar nomination for his memorable performance in “Forrest Gump,” as the paraplegic Vietnam War vet Lt. Dan.

“At the time I knew I was going to be playing Harry, I was going through all the Academy Awards stuff,” explains the droll actor over lunch in his suite at Pasadena’s Ritz Carlton Hotel. Despite the fact that the air conditioning is on the blink and the room is oppressively hot, Sinise seems remarkably cool.

“I got nominated for an Oscar,” he continues, chomping on his salad. “There are all of these different awards shows. I was so busy doing all of that and trying to prepare Harry Truman at the same time. I had to cram a lot into a short period of time, plus they had to start shooting by April 1, which was like four days after the Oscars.”

Not that Sinise, who stars in the summer box-office hit “Apollo 13,” is complaining about his good fortune. “I mean everything was great,” he says in his distinctive Midwestern twang. “But usually I would--especially for a character like this--have liked to have had a little more time before we started shooting. Just kind of a lot of time to think about nothing else but playing him. It was a daunting challenge to play somebody of such historical significance as Harry Truman. I had to keep researching it.”

One of the first things Sinise did was to spend two days at the Truman Library in Independence, Mo. “They let me sit in this little room and study film of him,” Sinise says. The first footage he saw was Truman’s speech at the 1948 Democratic convention, where Truman was fighting for his political life. More than 35 Southern delegates left the convention after Truman demanded and received a civil-rights plank in the party’s platform.

Immediately, Sinise called the producers to tell them that speech had to be included. “There’s this great moment when he comes out at 2 in the morning with a white suit and so well-put together when everybody else is all sweaty and grungy,” Sinise says. “He comes out looking so sharp and so strong because the Southern delegations have walked out. He’s very adamant and mad. It’s a very fiery speech. When I saw that speech I knew that I had made the right decision to play this guy. There’s a lot of fire in him.”

Pierson, who also directed the cable film “Lakota Woman,” says Sinise was “enormously helpful in the evolution of the script. It just kept growing and growing and growing. What can you say? It’s all there” in his performance.

Sinise knew little about Truman, who was a failed farmer, haberdasher, oil prospector, lead miner, banker, law student and county politician before assuming the Oval Office in 1945 after President Franklin D. Roosevelt’s death. Truman, who hated war, was the only world leader ever to use the atomic bomb.

“I had a picture of him in my mind when he was President,” Sinise explains. He was shocked to discover that Harry and Bess Truman never owned their own home and after he left the White House, in 1953, lived off his pension and the money he made from his memoirs and speeches.

“He truly was not interested in becoming a rich man,” Sinise says. “He was not good at business. He was really a public servant. That’s what he was good at--being a leader, gathering people, motivating them and getting them organized. He was really a representative of the people.”

And his relationship with Bess, whom Truman met when he was 6, was totally “pure,” says Sinise. “I spoke with some Secret Service guys who knew early Secret Service guys and the Secret Service knows when [the Presidents] skip out and stuff. Harry didn’t do that.”

Sinise has just completed filming the thriller “Albino Alligator,” in which he plays a criminal with a conscience. And he’s currently directing his first Steppenwolf production in two years, Sam Sheperd’s “Buried Child.”

Though he missed the theater, “I feel like my career is changing,” says the actor, who directed the feature films “Miles From Home” and “Of Mice and Men.” “It’s like a new part of my career these days with all of this. I spent basically from 1974 to 1990, with a few exceptions, focused on Steppenwolf.”

As for the “Forrest Gump” phenomenon, Sinise says he wasn’t shocked at the Oscar-winning film’s success. “Much like ‘Apollo 13,’ I think both the films say something real positive about human beings--how they can give each other something selflessly. They are both about unselfish human beings. We live in a very selfish time. Whenever people pull together to help somebody or save somebody, it’s heroic. We embrace it because we long for it.”

“Truman” airs Saturday at 8 p.m. on HBO.