“Never having a real speaking role before, I’ve been having to work on my, you know, my enunciation, my articulation, things like that,” said 27-year-old Matthew Risch, the luckiest ex-understudy in New York City. It was the last day of November, and he was sitting in front of a bulb-lined mirror in his new dressing room at the Roundabout Theatre’s Studio 54.

Nine days earlier, the actor Christian Hoff, 13 years Mr. Risch’s senior and poised for a breakthrough as the star of the retooled Rodgers and Hart mega-musical Pal Joey, injured his foot during a performance, according to an announcement issued by the theater. It was during a Friday night show, one week into previews.

“That’s why I almost threw up when they told me,” said Mr. Risch. “It wasn’t because of joy or nerves; it was because I felt so completely horrible for him, because he had been nothing but the most generous person I’ve ever had the chance to work with. I’ve understudied before, but I’ve never understudied a role of this magnitude. And he, from day one, came up to me and was like, ‘Anything you need.’”

The morning after Mr. Hoff’s mishap, his understudy was called in for a rehearsal as Joey Evans, the sleazy nightclub cad, a character Brooks Atkinson called “a rat infested with termites” when the show debuted, starring Gene Kelly, in 1940. And that night, Mr. Risch played the part for the first time in front of a paying audience. “It was a very frightening experience, but, my God,” he said, “it was exhilarating.” He was given the role for good the next day.

A week later, 15 minutes before a physical therapist was due to work on his hip, and an hour and a half before Sunday’s preview matinee, it was raining. Mr. Risch shuffled around the dressing room he had inherited from Mr. Hoff, wearing green-striped socks with pink toes and heels, jeans and a soft V-neck sweater that showed chest hair. His shelves had been stocked with two bottles of Cetaphil, Kiehl’s Facial Fuel, Tiger Balm (“which saves my life”), Simply Saline nasal spray (“also saves my life”), Red Cross table salt for gargling, Rosebud Salve, Axe deodorant, Brylcreem, two bottles of CVS brand hair gel and two bottles of Paul Mitchell spray. “I try and keep my hair in place as much as I can,” he explained. Nevertheless, he tends to pull at the pompadour-size tuft of hair that falls over his forehead and down to his massive, ready-for-close-up black eyebrows.

 

THE UNDERSTUDY'S TRIUMPH is practically the oldest story in show business. There’s the young Miss Harrington, triumphing (onstage, at least) over Margo Channing in All About Eve, of course. In real life, Catherine Zeta-Jones got her break when the star and the star’s understudy both fell ill for a performance of 42nd Street, which itself is about a star breaking her ankle on opening night. “All my friends have been reminding me that this happened to Shirley MacLaine back in the day, when she went on for … I should know this … Hold on, I have it right here somewhere,” Mr. Risch said, checking his iPhone. “Was it Can-Can or something? It was one of those oldies. … But anyway she got injured—oh, Pajama Game, it was Pajama Game—and there’s a story she was literally about to hand in her notice to quit and she got to the theater, and they were like, ‘Where have you been? You’re on!’ And she quickly put away her notice, went onstage, the producers were there, they offered her the part, and the rest is history.”

In Rosemary’s Baby, Mia Farrow’s horrifying husband, played by John Cassavetes, gets a big part because his competition is mysteriously blinded. Do understudies—unconsciously even—wish evil on the leads? “No! I hope not. I certainly didn’t,” Mr. Risch said. “And I don’t think—the good people don’t. I consider myself a good person; I would never wish harm on anybody ever. I believe in karma.”

Bob Fosse understudied for Harold Lang’s Joey during the long-running early-’50s revival, but never got on. In another revival 11 years later, Fosse finally got the lead role, plus a Tony nomination.

“I had been very back and forth,” Mr. Risch said, “with, ‘Am I going to go on?’ ‘Oh, I bet I’ll go on.’ ‘Oh, I don’t know if I’m going to go on.’ Christian is a very consummate performer, and I knew because it was a limited engagement, I was like, ‘Oh, I don’t know if I’ll ever get a chance to go on.’ But, you know, I knew his wife was pregnant, so I was like, ‘Maybe she’ll go into labor early and I’ll get to go onstage!’”

Mr. Hoff couldn’t be reached for comment.

“I sent him an email on Thanksgiving,” his former understudy said, “and I haven’t heard from him, but I know he’s healing and doing well and with his family.”

 

MR. RISCH GREW UP in Massachusetts. “I had the best childhood. Oh my God, I had a great childhood, absolutely. It was fantastic,” he said. He went to the artsy Tower School in Marblehead until ninth grade, where he was excused from gym because of his theater work. “I would have had to do sports, and it freaked the fuck out of me. It really scared me. I had a huge fear of sports and jocks. I was bullied as a kid by Marblehead local kids every once in a while—Chris something, Sadulski? Asshole.”

By middle school, Mr. Risch said, “I was doing just, like, every community theater production I could get my hands on. I never played a role; I was just always in the ensemble. And I loved it. I loved theater. I loved it so much. I became obsessed with musical theater. I was a show queen from a very young age.”

At Walnut Hill high school, his first role, in 10th grade, was Robert Conklin in The Rimers of Eldritch. “There’s this scene at the end where he rapes this girl, and I remember the audition was doing the rape scene. And I just kind of went for it, and I think the director was kind of enamored with how gutsy I was.” In August 1998, The Scotsman called Mr. Risch “riveting” in A Chorus Line.

But his four years at the University of Cincinnati College-Conservatory of Music were a bit of a letdown. “I didn’t love it there, to be completely honest,” he said. “I don’t want to say ‘ahead of everyone else’?”

He graduated in 2004 and moved to New York. By 2005 he was understudying a supporting role, Fred Casely, in Chicago; then came Legally Blonde, where he was the second understudy for the part of Warner Huntington III. He left in late summer for Pal Joey. “So this was exciting for me to be the only understudy, you know?” he said. “I took the responsibility really seriously.”

Mr. Risch was sitting in front of the bulb-lined mirror and using a tissue to swat at a fly, but missing. “I think everyone’s main concern is that—I don’t want to sound like I’m bragging about myself or anything—but I tend to be a nice guy. I’m not really an asshole womanizer type. And Joey’s a real punk, a real street rat,” he said. “He treats people roughly and is an ass.”

Almost two decades after Gene Kelly went for verminlike loathsomeness, Frank Sinatra did a zippier, hipper Joey in the 1957 film, co-starring Rita Hayworth. “I honestly fell asleep to it, I thought it was so terrible,” Mr. Risch said. But Kim Novak is in that film, too. “She’s awful! The second she came on that screen I was like, ‘What! Is! She! Doing!’ I just thought she was such an awful actress in that film; I feel so crazy saying that, but I really did.”

His Joey will be mucky and grubby but sympathetic, he promised. “‘He’s just spewing out all this vomit of charm,’” Elaine Stritch, the legendary 83-year-old actress (who was in the 1952 revival), advised Mr. Risch while visiting the theater a few nights prior. “‘Vomiting up charm left and right to everybody, it’s nothing to him. That’s all he knows how to do.’”

Co-stars Stockard Channing and Martha Plimpton came to talk, too. “They’re like, ‘We’re so happy you’re doing this. We couldn’t be more pleased. We’re here for you. Whatever you need, let us know,’” Mr. Risch recalled. “I’m like”—he paused and exhaled hugely—“‘Thank you.’”

Commenters on sites like BroadwayWorld.com have also offered enthusiastic support for the unexpected cast change.

Opening night, meanwhile, has been pushed back from Dec. 11 to the 18th. “They could have hired a celebrity—I think they would have had to push back the opening a lot further than a week,” Mr. Risch said. “They gave me a week, which I told them I didn’t even really think I needed.”

Is he nervous that audiences may hate his slimy character? “Oh, absolutely, but that’s the challenge,” said the erstwhile understudy. “You have to get them from that first scene. You have to get them when he’s auditioning and all he wants is a job. He just got thrown out of a club. He’s gone from town to town. It’s all about achieving his goal. And making his dream come true, making his dream a reality. How can they not sympathize with that, though? How can a New Yorker especially come to see a show about a guy whose only dream is to own a nightclub and to make people happy? That’s all he wants to do. And how can you at the end of two and a half hours really hate a guy for doing that? Well, the way he goes about it is pretty crummy and pretty slimy, but at the end of the day it’s all about his drive and ambition.”

mabelson@observer.com