Kevin Fox is understudy for the title role in the musical “Buddy: The Buddy Holly Story” at the Walnut Street Theatre. (John Costello / Inquirer Staff Photographer)

Kevin Fox slips on the thick black frames, and the fact that he's 6-foot-1, 200 pounds, and about 20 years too old doesn't matter anymore.

He is Buddy Holly, and an hour before the curtain rises, he's working on his second glass of beaujolais.

When Fox returns to the Walnut Street Theatrethe standard 30 minutes before show time, no one will be waiting for him. He need not check in.

He's had to master every line and gesture, the lyrics and guitar licks to 21 songs, knowing he'll probably never play them for anyone other than the six crew members who caught this afternoon's rehearsal.

He is the understudy.

And halfway through the seven-week run of Buddy: The Buddy Holly Story, there's talk in the theater's green room that leading man Christopher Sutton might want to hire a food taster.

"People see me in the glasses and they say, 'I think you're taking this gig a little too seriously,' " Fox says, sliding the specs into the pocket of his denim shirt and sawing into a medium-rare filet mignon at the Caribou Cafe.

They're the actor's motorcycle goggles, really. He found them in a New York bar a few years ago, and they keep the bugs out of his eyes when he's riding his Honda Silverwing.

Taking the job of Sutton's understudy made little professional sense, says Fox, a Vancouver native who lives in Soho.

"I have the pressure of doing the entire show should he be sick, knowing that he probably never will be. He's a seasoned professional, an easygoing guy. He takes care of himself."

Understudies typically don't rehearse until the show has opened. If Sutton had taken ill early in the engagement, Fox would have had to walk on stage cold, facing as many as 1,075 people without a single run-through under his belt.

But now, he's prepared. Ridiculously prepared. And he's looking for more than his nightly gig playing the fourth member of Holly's band, the Crickets.

"Once I knew I was ready to go, I said, 'It's time to have Chris over for dinner,' " he says, suddenly sounding more like Vincent Price than the ill-fated rock-and-roll pioneer.

Memorizing lines wasn't Fox's biggest problem: He was an understudy in Buddy's 1990 pre-Broadway tour and New York run.

But that time he was just a spare Cricket who stepped in if the real Cricket was called upon to take over the lead. Sort of like being runner-up to the runner-up at the Miss America pageant.

When his agent called to tell him he was understudying the lead role, "first thing I did was quit smoking," he says. He was up to a pack and a half a day. "The next thing I did was start to lose some weight." He shed 15 pounds. Then he rented an electric guitar. He'd played since his early 20s, but never outside his living room.

Performing comes naturally to Fox's family. One brother, musical director of Rent on Broadway, also plays piano at clubs, and when he can't make a date, his mother sits in. Another brother is a songwriter in Nashville. A third has been in Phantom, Show Boat and Ragtime in Toronto. His father was a Canadian radio personality.

One line from the play jumped out at him as he prepared this summer to be the replacement Buddy: "Just 19 years of age and so much talent."

Fox is 37. "I said, 'Oh, man, 19. I hope it's a big theater.' "

It's not a role with much future for him. No one would cast him as Holly, he says. "I thought, 'I'm going to learn all this and for what?' It's going to be an enormous learning experience, which will get me nothing, except I'll be a lot better guitarist, which isn't bad."

Still, he likes working close enough to New York to go home a couple of times a week - he has a photography business there. And then there's the pay: As an understudy, Actors' Equity Association guarantees him a whopping $27 over his Cricket pay every week. He negotiated for considerably more, he says.

"My favorite understudy story?" muses Fox. "Jack Palance was doing Requiem for a Heavyweight at Lincoln Center. He was also the understudy for Brando in Streetcar Named Desire. He'd go to Lincoln Center to do his show and if Marlon chose not to go on - which he did regularly - Jack would have to go do an equally heavy role and his [Requiem] understudy would go on for him. Man, that's the life."

Fox is tanned and relaxed, which is surprising, given his last 24 hours. After two performances of Buddy the day before, he took off for Manhattan at 11 p.m. on the Silverwing. He made it to the lab just in time to pick up negatives he needed for a job.

At 4:30 a.m. he was done in the darkroom, where he tinted 8-by-10 black-and-whites for an actress doing a one-woman show. He slept until 11:30 a.m., turned the work over to the client, had coffee, then biked back for a 3 p.m. rehearsal, the Philly engagement's one and only full run-through for understudies.

"I'm dying to go on. I'd love to," says Fox, who's had small roles in the films Hearts of Fire and The Big Town. "I'd feel like I've worked for something. It's a little bit empty." Still, he has a good relationship with Sutton, who attended the rehearsal at Fox's invitation.

"A lot of understudies look at the lead actors with daggers. No question I'd like to play that role. But if I were a money man and was going to finance this show, I'd back Chris."

Five minutes until he's due at the theater. "They don't take too kindly to you being late," he says, getting up and bounding down Walnut Street, talking the whole time.

"I love the double life," he says, approaching the marquee. "When I get sick of some director telling me every breath to take, I go and I'm my own boss."

There's a crowd outside the theater, older couples generally, who can remember when hits like "Rave On" and "Peggy Sue" shined on the late-'50s radio.

The understudy blends into the crowd, his glasses tucked away in his pocket. No one pays him the slightest attention.