If Christian McKay seems uncannily accurate in his riveting portrayal of Orson Welles, the legendary star of stage, screen and radio whose outsize personality was as much a part of his mystique as the productions he so meticulously crafted, credit the man with doing his homework.

A graduate of London’s Royal Academy of Dramatic Art, McKay, who stars in Richard Linklater’s Me and Orson Welles, bristled the first time he was likened to the Wisconsin-born auteur, recalling not the handsome, electrifying youth who directed and starred in Citizen Kane at age 25, but rather the bloated, weary pitchman of Welles’ later years. McKay (rhymes with “pie”) assumed being called Wellesian was a knock on his weight.

Since then, McKay, 36, has learned to embrace the comparison. He has studied Welles exhaustively, reading nearly 100 accounts of his life and career. His one-man show, Rosebud: The Lives of Orson Welles, earned rave reviews in both Edinburgh and New York. Now, McKay is taking his famous alter ego to the screen in Linklater’s comedy, which finds a 22-year-old Welles readying his landmark stage production of Shakespeare’s Julius Caesar.

“It was an extraordinary challenge to portray Welles at the beginning of his career,” says McKay, a Lancashire native. “I’m the only actor who had to lose weight to play him.

“I’ve read that you can go as large as you like in the theater, but on film you have to be contained. Well, I was playing a theatrical, larger-than-life figure – I couldn’t very well bring him down. Richard said he could teach me how to capture that personality on camera without sacrificing any of the authenticity, but the character I ended up playing felt very different from the one I played on stage.”

McKay was recommended to Linklater by Robert Kaplow, the high-school English teacher whose bestseller inspired the film. Linklater flew to New York to see McKay in action, then summoned him to Austin, Texas, for screen tests. He knew he’d found his Welles – the only challenge left was convincing his backers that the little-known actor was right for the role.

“I was given a list of American stars to look at, and I don’t want to single anyone out, but none of them was suited to playing Welles,” says Linklater, 49. “It’s a challenge to cast an unknown, but that’s where the magic comes from. By not knowing Christian, it’s possible to believe that you’re watching Orson Welles putting on Julius Caesar in 1937.

“I think Christian has now scorched the earth when it comes to Welles portrayals. For someone to play him now would be like trying to play Ray Charles in a movie [after Jamie Foxx]. He’s got that role locked down for a generation at least.”

McKay, recently named Best Supporting Actor by the San Francisco Film Critics Circle, seems tickled by the compliment – he’s been hearing it a lot lately – but worries about being typecast. Case in point: A producer recently approached him about playing Harry Lime, the character immortalized by Welles in Carol Reed’s The Third Man, in a follow-up cleverly titled The Fourth Man.

McKay politely declined, and finds his options less limited than they once were. He’ll be appearing in one of Woody Allen’s next movies (“I haven’t a clue what it’s about,” he says with a laugh) and is currently working on several screenplays. For that, he thanks Linklater, whom he credits with teaching him all he knows about acting for the camera. But, like Welles, he remains a dedicated student of the stage.

“People keep telling me, ‘This is going to change your life!’ Well, I don’t want it to change my life,” McKay says. “I’m loving the experience, but I have a very happy life. And I’ve learned the lesson, by playing this wonderful maverick, that if the phone doesn’t ring, I don’t have to wait for it.

“Suddenly, the stage plays I’ve been working on have become screenplays. I’ve fallen in love with making film – it’s the biggest electric train set a boy was ever given. But I am no less passionate about the stage. I still believe theater is the best training ground for actors, the essence of what performing is all about.”