When Tracy Letts leaves town in a few days, New York will find itself short one talented playwright – and one talented actor. The 39-year-old Oklahoman is best known to New York audiences as the author of the audacious, grisly off-Broadway hits “Bug” and “Killer Joe.” But around the country, he has a second persona, as a gifted performer. He played George in “Who’s Afraid of Virginia Woolf?” in Atlanta. He has done Richard Greenberg and David Mamet plays in Chicago. He recently earned a place in the prestigious Steppenwolf ensemble there.

This spring, “Orson’s Shadow” has given New York its first glimpse of Letts-as-actor, and the reaction of many in the Barrow Street audience is, Where has this guy been? Austin Pendleton’s play concerns the tumultuous 1960 London premiere of Ionesco’s “Rhinoceros.” As the star Laurence Olivier and the director Orson Welles do battle (joined by Olivier’s co-star/lover Joan Plowright and his unstable wife Vivien Leigh), Mr. Letts serves as beleaguered referee. He plays the critic Kenneth Tynan, who organized the production and now narrates its mishaps to the audience.

Knowing Mr. Letts only through his Grand Guignol plays from the seedy heartland, I was surprised to see how effortlessly he slipped into a brown suit and trench coat to play the chagrinned British critic. Over lunch recently in Park Slope, where he has been staying with a friend while in town, that initial surprise was compounded: In his black T-shirt and slight Plains-state drawl, you wouldn’t peg him as a theater person at all.

Paradoxically, it’s that self-effacing quality that makes him such an outstanding theater person, both as actor and playwright. “I’m not a fan – this is a personal preference – I’m not a fan of tour-de-force writing,” he said. “I admire it, but it’s not where my inclination is. I want to hide.” There are no obvious author surrogates in his plays; no characters pop up to deliver the gospel according to the playwright. He recalled a long speech he wrote for Peter, the deranged protagonist of “Bug.” “It was some of the best writing I ever did, but I cut it because it was drawing attention to itself,” he said.

You can see how this vanishing impulse might prove tricky for an actor who, in “Orson’s Shadow,” literally stands in a spotlight, alone onstage, addressing an audience of hundreds. Tynan is, in Mr. Letts’s phrase, our “tour guide” through the show, giving us some background and explaining what’s at stake for the characters. It’s not the first time Mr. Letts has gone toe-to-toe with a crowd – he has played Tom in “The Glass Menagerie” twice, for instance – but still he finds it “nerve-wracking.” “It’s a dialogue played with the audience. As such, it’s hard to do – unpredictable.”

On his first trip here as an actor, Mr. Letts hasn’t detected any sharp differences between New York and Chicago audiences. “Cell phones go off in both places,” he explained. “It’s getting to be a real problem. We do eight shows a week, and a cell phone will go off six times. I told a producer friend that he should charge $150 per show, but only $50 if you check your cell phone at the door.”

Over the course of a long run, New York audiences traditionally shift from theater insiders to civilians more generally, but Mr. Letts said the play’s reception hasn’t changed. There are, of course, the inevitable variations from one audience to the next. One night recently, he delivered his first soliloquy, a funny speech setting the stage for the show, and got no response at all. “You want to tap on the glass and say, ‘Anybody out there?'” he recalled. “But we’ve learned to trust that a quiet audience doesn’t necessarily mean they’re not enjoying it. Sometimes we talk to them after and find out they were rapt.”

A second visit to the show, two months after its opening night, revealed that the actors’ performances have grown somewhat broader over time (they usually do): The battles are noisier, the fights are dirtier than early in the run. But Mr. Letts’s speeches remain little marvels of disillusioned wit. In the play, Tynan describes Welles as a father figure and Olivier as a hero. Mr. Letts lends the soliloquies a kind of despairing hope, full of affection laced with disappointment; he knows too well his friends’ great abilities and their flaws. The really remarkable thing about Mr. Letts’s performance is how strong an impression he makes with only a handful of speeches. After the first viewing, I had remembered him delivering many more.

Mr. Letts’s portrayal seems entirely sympathetic to the emphysema-riddled Tynan, but how does he feel about playing a critic, one of the damned? “I hate them all,” he said with a smile. “No offense.” A playwright friend of his couldn’t figure out how Mr. Letts was able to play a critic, since the friend finds the entire profession reprehensible. Mr. Letts doesn’t go that far. “I just find most people in the position reprehensible,” he said with another genial grin.

Even when playing characters in pleasanter lines of work, Mr. Letts doesn’t like to do much background work. He did watch Tynan’s interview with Olivier in the Criterion DVD of Olivier’s “Richard III.” And Jeff Still, who plays Welles, gave Mr. Letts a short travel film that Welles made in Spain, starring the young Tynan and his wife. “Peter Bogdanovich saw the show. Afterwards he said, ‘You really nailed him, really got his essence.’ I said, ‘That’s funny, Peter, I did no research.'”

Mr. Letts gives his last performance on Sunday; his replacement is Pearl Theatre regular Sean McNall. This summer, William Friedkin will film Mr. Letts’s screenplay of “Bug” (starring Ashley Judd and original lead actor Michael Shannon). His most recent play, the Pulitzer finalist “The Man from Nebraska,” will be produced in about ten theaters around the country next year. But inexplicably, this sorrowing tale about a man’s crisis of faith still hasn’t found a New York producer. He has, in fact, no offers from New York of any kind: No playwriting commissions from the city’s theaters, no calls to act in their shows.

New York’s loss is Chicago’s gain. He will appear in three plays in the upcoming Steppenwolf season, including the latest from Richard Greenberg. After a short vacation, he’s headed there to begin rehearsals on the season opener, and sounds pleased at the change of pace. “It’s a contemporary play, set in America,” he said. “I’m not the lead. I come in, sit on the couch, and crack jokes. I can’t tell you how much I’m looking forward to it.”