It started as the kind of goofy routine actors do for each other in the dressing room. For Mark Setlock, it was scenes from "It's a Wonderful Life."

Setlock was in Portland, Ore., doing "Fully Committed," which is by Becky Mode but based on characters developed by her and Setlock. (That's the one-man show about a day in the life of the phone reservationist at a snobbish New York restaurant, staged in 2002 at the Public Theater.)

The Portland company was looking for a Christmas show to vary a diet of "The Santaland Diaries." So they suggested Setlock might tackle a solo "Wonderful Life," and since he loves the movie and has seen it so many times he pretty much knows the whole thing by heart, he figured, why not?

And that's how (with a few steps in between) Setlock finds himself in Pittsburgh now, doing preview performances at the Pittsburgh Public Theater of a one-man version of what is called "This Wonderful Life."

The set-up is very simple: an actor (Setlock) appears on stage and basically tells us, "I love this movie, so I'm going to do the whole thing for you by myself."

It sounds crazy, but that's part of the appeal. Setlock is a compact actor with a face that is saturnine at rest. But acting is about transformation, and as he talks, his face brightens. The magic of the stage is such, abetted by Setlock's skills and the audience's willing powers of imagination, that the play works -- judging by the audience response he's experienced.

When the idea was proposed, Setlock asked for a writer to work with, as he had worked with Mode on "Fully Committed," and the Portland theater found him Steve Murray. "I just needed someone to structure it," the actor says, "to pick out the best bits, because I love it all."

In the show, he does some commentary on the movie and the social aspect of the times, but he and Murray discovered it doesn't take much. Basically, Setlock just reenacts the story, moving instantaneously from one character to another. He says it takes about 10 minutes for audiences to accept the oddity of it and get caught up in the story. The story is so strong that even people who've never seen the movie -- apparently, there are some -- are moved by it, while those who do know the movie can imagine the whole setting.

In Portland, he did it with a big, realistic set and a period costume, but here they have opted for "a simpler set, more like I had in mind -- an actor walking into a theater and using the things I find." There are some set pieces, though, including a bridge.

Setlock says he isn't Rich Little, doing impersonations: He just channels the movie in 70 minutes. "If I really sound like anyone, it's Potter." Asked about his Donna Reed, he laughs. About Violet Bick, the town floozy, he says, "Playing a woman with no costume is hard."

This is only the second time he's done his show. Other actors have done it a couple of places since Portland, and he did a few readings in pursuit of a New York production, but nothing's come of that yet.

"In Portland, they asked me to come back, but I said I didn't think I was going to do it again. It's exhausting." But when the Public's Ted Pappas approached Martha Banta, who had directed him in Portland, she asked Setlock and he agreed.

Growing up in Cleveland, Setlock discovered theater when he appeared in "Arsenic and Old Lace" as a high school freshman. He went to Kent State, then did a two-year graduate program at the American Repertory Theater at Harvard.

Trained and raring to go, he arrived in 1992 in New York. It took New York a while to catch on -- it was five years before he had a paying theater gig. Then he was in a workshop of something called "Rent," which eventually took him to Broadway as an understudy and then an ensemble member.

Meanwhile, he was working with Mode on "Fully Committed," which tried out at a theater in the Adirondacks run by director Banta, then debuted in 1999 off-Broadway. Setlock left after nine months to do it in Los Angeles and later for eight months in London. He was succeeded in New York by Roger Bart and later by Christopher Fitzgerald, both of whom are now in Mel Brooks' "Young Frankenstein."

In other words, "Fully Committed" has been quite the meal ticket. But that doesn't mean Setlock is in love with doing a solo show.

"After 'Fully Committed,' I said I'd never do it again. It's a lot of work, and you're lonely. It's just more fun to be with people."

There's also a lot of pressure. "It's easy to take it personally if they don't seem to enjoy it. But the least vocal audiences can be most appreciative at the end, so you learn just to keep going."

For a similar experience, he saw Patrick Stewart's solo "Christmas Carol," and it confirmed what he knew: "If you commit to it, to the apparently silly idea of carrying on conversations with yourself, then people will buy it. They want to get lost in the make-believe."