His name is Mr. Lockhart. Beyond that, we can’t—nay, won’t—tell you who Tom Irwin plays in The Seafarer. To do so would destroy one of the most deliciously skin-prickling secrets of the season. Suffice to say, Mr. Lockhart isn’t anything like the well-meaning priest Irwin plays on TNT’s Saving Grace.

"There’s a lot of remorse and anger with this guy," Irwin says of his Steppenwolf character, a stranger who joins the boozy Christmas Eve poker game at the heart of Conor McPherson’s lacerating comedy. "He’s been profoundly rejected, so you feel for him, even though—" and here Irwin breaks off. "Look, I don’t want to say any more. Except that he’s got this monologue. . . ." Does he ever. At the apex of a long, menacing revelation, Mr. Lockhart unleashes an utterly unnerving verbal vortex that ends in a black hole of terror and despair.

A Peoria native who’s now based in Los Angeles, Irwin, 52, is one of those actors you instantly recognize, even if you can’t quite place them. He’s been featured on Judging Amy, Numb3rs, The Closer, ER, and My So-Called Life (playing Claire Danes’s father—TV Guide named Irwin as one of its top TV dads of all time). He’ll show up on Lost in the near future (that role is also a closely guarded secret). All this work means he doesn’t get back to Chicago much. Despite being a Steppenwolf ensemble member since 1979, he hasn’t graced its stage in five years. "If theatre paid as well as movies and TV, I’d do it all the time," he says.

Monetary concerns took a back seat after Irwin caught The Seafarer on Broadway last March. He was on the phone with Steppenwolf’s artistic director, Martha Lavey, the next day. "I basically had one question for her—how could I be in this show?" he recalls.

He couldn’t. The cast was pretty much set. Fellow ensemble members John Mahoney, Francis Guinan, and Alan Wilder were in. And Tracy Letts had been offered the role of Mr. Lockhart. "Martha told me that if Tracy wasn’t interested, I  could do it,"  says Irwin. Ultimately, Letts got busy with his soaring August:  Osage County and turned the part down. (August beat The Seafarer for 2008’s Tony for best play). So Irwin headed for Chicago. "This is a part that you’ll never get bored with. He’s someone I can sink my teeth into," Irwin says. "Mr. Lockhart—he’s truly horrifying. I  thought, How much fun would it be to go there? Especially on Christmas Eve?" Hmm. Maybe Tom Irwin’s not so nice after all.

 The Seafarer runs Dec. 4th–Feb. 8th at Steppenwolf, 1650 N. Halsted St. For tickets, 312-335-1650; steppenwolf.org

Photography: Scott Council