Six performances a week, wearing dashing turn-of-the-century costumes and singing a score that, for all its recognizability, is no cakewalk.

Agent Jack Bristow — er, make that Victor Garber — are you game for the assignment?

"I’ve been working my butt off," confessed Garber, who headlines the production of "A Little Night Music" beginning performances tonight at the Dorothy Chandler Pavilion. "This music is really hard and it’s very important to me that I feel confident and good about it."

The score to "A Little Night Music" is arguably Stephen Sondheim’s most accessible and among his most frequently performed work (who doesn’t know "Send in the Clowns"?). With a book by Hugh Wheeler and based on Ingmar Bergman’s film "Smiles of a Summer Night," "Night Music" is a Swedish roundelay of current and ex-lovers, coupling and uncoupling during a weekend gathering in the country.

Garber plays Fredrik Eggerman, an attorney and former lover of the actress Desiree Armfeld. Fredrik is now married to a much younger woman, Anne. Fredrik is a role — and "Night Music" a show — that the London, Ontario, native Garber says he has long coveted.

In the version staged by director Scott Ellis for New York City Opera last summer, Jeremy Irons — an actor not generally known for his vocal abilities — played Fredrik. Irons and co-star Juliet Stevenson were announced to reprise their roles for the monthlong L.A. Opera run — the first summer production presented by the company.

Judith Ivey replaced Stevenson and when a film role for Irons threatened to overlap the production schedule, Irons dropped out and Garber stepped in, much to the delight of longtime friend Ellis.

"He can do it. He can sing it and act it," Ellis said of Garber, whose theater credits include Tony-nominated roles in "Little Me" and "Damn Yankees." "And Stephen (Sondheim) has always wanted Victor to do this role."

"I’m the right age now, and it worked out perfectly," added Garber, who at 55 sports a thin mustache shot through with gray and a shock of curly hair to match. "The score is perfect. Everything in it is immaculately done, the story is magical, and I think it’s the most romantic and lush musical of Sondheim’s canon."

Having created roles in two Sondheim musicals, Garber should know. He played Anthony Hope in "Sweeney Todd" in 1979 and John Wilkes Booth in "Assassins" more than a decade later. There was also a workshop production of Sondheim’s "Wise Guys" with Nathan Lane and directed by Sam Mendes that would serve as a rough prototype for the composer’s failed latest work, "Bounce."

Actor and composer speak with some regularity and meet at various functions. Garber called Sondheim after seeing the recent Roundabout Theatre revival of "Assassins" that earned Michael Cerveris a Tony award for playing Booth, the role Garber created.

"He’s very excited about this production and hopefully he’ll come out and see it," Garber said of Sondheim. "We’ve done a lot together and I feel privileged to be alive at the time he’s writing theater because there’s just no one better."

The occasional benefit notwithstanding, Garber has been absent from the stage — musical or otherwise — since he reprised his role in the Los Angeles run of "Art" with fellow co-stars Alan Alda and Alfred Molina in early 1999. In the ensuing years, he has played Inspector Philip Millard in a pair of TV films, taken on a sleazy law professor in the original "Legally Blonde," and earned his first Emmy nomination for playing Sid Luft in "Life With Judy Garland: Me and My Shadows."

And, oh yeah, he also joined a hit show, playing spy dad to Jennifer Garner in "Alias" and picking up another Emmy nomination in the process.

"Alias" creator J.J. Abrams knew Garber’s work before casting him for the show’s pilot episode. Garber’s film and TV work prior to "Alias" has been, if not always showy, usually memorable: characters like Thomas Andrews, designer of the ill-fated liner in James Cameron’s "Titanic," or Goldie Hawn’s ex-husband in "The First Wives Club."

"Having known his work for a long time, and having been a fan, when I saw he was potentially available to do the series, it was sort of done for me," said Abrams. "I just thought he was just a genius. And barring any character issues in terms of who he was as a person. As an actor, he couldn’t be beat, and he turned out to be as nice and sort of good-hearted a guy as he is a brilliant actor."

A humble actor, too, the executive producer discovered, much to his delight.

"Once we were doing a scene where the two of them were driving in a car. I was in the camera car and Victor and Jennifer Garner were in the car behind and I could hear them talking between takes," recalled Abrams. "Victor said to Jennifer, ’I don’t know why he thinks I can play the part of a spy, but I trust the little man,’ which made me laugh.

"It was funny to me. He was so convincing as Jack so quickly, but even in the midst of shooting it, he had the insecurities. He certainly didn’t show it."

"Alias" means limited time for other projects, which can only happen during show hiatuses. But Garber, who has discovered that he misses the time spent in the rehearsal room, has no complaints. He’ll be a lawyer in love this summer and then he’ll happily go prime-time covert again.

"I owe J.J. Abrams everything, and I tell him that constantly," said Garber. "He’s given me another life, and a good one."

A LITTLE NIGHT MUSIC Where: Dorothy Chandler Pavilion, 135 N. Grand Ave., L.A. When: 7:30 p.m. Wednesdays and Thursdays, 8 p.m. Fridays and Saturdays, 2 p.m. Saturdays and Sundays; through July 31. Tickets: $20 to $100. Information: (213) 365-3500.