They called him "Mr. President" on the film set, and that's how Frank Langella wanted it.

It wasn't a matter of ego. Rather, it was his need to remain immersed in the brilliant but tortuous mind of Richard Nixon.

Langella had already portrayed the disgraced U.S. president hundreds of times on stage in the award-winning Frost/Nixon, and now that Peter Morgan's compelling play was being turned into a film by director Ron Howard, the 68-year-old actor was determined to keep his Nixon fresh and truthful.

"I came out of my trailer always as Nixon," he tells reporters. "I never broke character on the set. The company related to me in that way."

So, every day the assistant director who would walk him to the set would greet him with a "Good morning, Mr. President."

This was not one of those projects where Langella would walk onto a set and say to his fellow actors: "Hi guys? How ya doing?" He and Howard set different ground rules early on.

"I said to Ron, 'I think maybe it would be a good idea for me if I stay inside the character when I'm on the set, so that people react to the president differently than they would react to anyone else.' So, the moment the trailer door opened, I was Nixon until I went back again. And that was very helpful to me and very helpful to the company ...

"Ron had obviously spread the word ... so for those 40 days or so, it was really wonderful. It's good to be the king."

But this was a flawed king - in the eyes of his detractors... the epitome of evil by the time he resigned the presidency in 1974 under the shadow of the Watergate scandal and looming impeachment proceedings. Frost/Nixon is award-winning British dramatist Peter Morgan's take on events surrounding the famous 1977 television interviews between Nixon and English television superstar David Frost.

The interviews kept 45 million American viewers in suspense over four evenings as Frost attempted to lure a wily and stone-walling Nixon into admitting to the abuses of power that forced his resignation.

Both Langella and Michael Sheen, who plays Frost, received a cluster of awards for their original stage performances in both New York and London, and now with the film starting to open, there's Oscar talk in the air for both of them. But neither felt their casting in the movie was a certainty because they were not star names.

Although Howard wanted them both to recreate their roles for film, he had to persuade the Universal brass first.

"You might say I had a year-and-a-half audition for the movie," Langella says wryly as he remembers all those months playing Nixon onstage. "I heard every name except Dolly Parton mentioned for this part."

But finally, following the successful transfer of Frost/Nixon to Broadway, Langella received the call from Hollywood. And then he proceeded to prepare for the movie by doing - nothing.

"Because I had lived with him for such a long time, I thought that if I do anything before I walk in front of a camera, it's going to be a layer over what I've done already. So the moment the play closed, I forgot about him and didn't even open my mouth as Nixon until the first day on the set."

The slender actor with the close-cropped white hair and the charcoal pullover bares no resemblance to the hulking, swarthy ex-president of the stage play and film. It's difficult as well to imagine Langella as 16th-century English martyr Sir Thomas More - but he's currently doing that, to critical acclaim, in a Broadway revival of A Man for All Seasons. But his chameleon-like ability to bury himself in a character is nothing new.

This is an actor who has also succeeded with Dracula and Henry Higgins in My Fair Lady.

At the very beginning, when he first knew he would be entering Nixon's world, Langella spent hours at the Museum of Radio and Television watching old tapes. He talked to TV personalities like Mike Wallace and Barbara Walters - "anybody I could think who knew him, talked to him, worked with him.

"I absorbed as much factual information as I could. But then, once you start playing a part, all that should really go out of your head and you should begin to get the essence of the man. So, if you will have it, the second half of the preparation was much more about uncovering his soul for me and finding a way into it."

Usually, Langella and a character part company quickly once he's finished the acting job. Nixon is different. He continues to linger in Langella's mind.

"The depth of this man's pain and the depth of his desire for greatness is what came to me more than anything else. The continued suspicion and paranoia stayed with me.

"He was always around in my head, and still is. I don't mean I live my life consumed by him, but the particular things that drove Nixon are in all of us. He just happened to have more of it than anybody else. I think that's why we are so fascinated by him and why he makes people cringe. He makes us think, 'Oh, I have those qualities, but I try to hide them.' He couldn't."

In a year when the competence of George W. Bush is being questioned by virtually everybody, Langella wants to make one point clear: Nixon was brilliant.

"He had a great mind and he had great aspiration towards doing great things for the country," Langella says. "He was brought down, I think, by his own personal inability to stay on top." The actor thinks Nixon was haunted by a "deep, dark side" that kept saying: "I don't think I belong here."

Langella doesn't feel surprised that his evaluation of Nixon the man is so complicated.

"My approach is to find the soul of a person first," he stresses. "I'm not a good mimic or a good imitator."

And he learned something from playing Nixon - compassion for human fallibility.

"He taught me more than any other character I've played ... never to judge anyone, never to really condemn anybody and never to define a person by a single action or even a crime. His crimes were clear. He was guilty of what he did. He should have resigned. He was clever enough not to get himself impeached.

''But there are so many other things about him - his childhood, his three brothers, his inability to function with girls, his feelings of always being inferior, of always wanting to get ahead of the basketball captain. He taught me that it doesn't really matter how you rise in the world or what position you get or how much money you achieve. He reminded me to look at people just as people."