Glamorous Desiree (Holly Twyford) faces her past in “A Little Night Music.” (Paul Tate Depoo III)

Very few people end up with their high school sweethearts. In a way, Eric Schaeffer did.

“The first [Stephen] Sondheim musical I saw was ‘Sweeney Todd’ on Broadway, the Angela Lansbury one,” says Schaeffer artistic director and co-founder of Signature Theatre. “Our high school drama class went up and we watched the show and I thought, ‘What was THAT?’ I went to the record store and bought the album — which I still have — and I just sat for hours listening to it and I was just like, ‘Oh, my God, this is brilliant.’ And that literally got me hooked on what a genius he was.”

You could say he’s still hooked. Schaeffer, 54, has directed 28 Sondheim shows at various theaters, including “Sweeney” four times. He memorably mounted it for Signature in 1991 as the theater’s first-ever musical.

“We did it in a space that had 80 seats,” Schaeffer says. “We had 16 in the orchestra, 18 in the cast and people were like, ‘How are they even going to do this? This is ridiculous. Who is this theater company?’ And then we did it and people were like, ‘They really know how to do it.’ ”

Schaeffer and Signature have established a national reputation for excellence in Sondheim — one Schaeffer hopes to continue with “A Little Night Music,” which began performances this week. Set in 1900 Sweden, it’s the story of a variety of people confronting a variety of sexual situations: There’s a married virgin, a sex-starved student and a lusty maid, for starters. And though Schaeffer is a Sondheim guy, this particular Sondheim, which premiered in 1973, is a first for him. He elected to take it on now after hearing Bobby Smith, who plays central character Fredrik, sing the show’s “Now” in concert.

“I immediately thought, ‘We need to do “A Little Night Music” with him,’ ” Schaeffer says. “That’s really the trigger that started it all.”

Schaeffer also feels he’s better equipped to handle the show now than during Signature’s previous staging. “It’s [about] people in the wrong relationships,” he says. “I guess I’ve been in some wrong relationships in my life, and now I’m in the right one. There’s no doubt that I’m older, wiser and totally get what the show deals with on a level that 20 years ago I couldn’t.”

Still, he tries to embody the spirit of Signature’s early days in his work. “It’s that carefree spirit of ‘let’s try it, let’s do it,’ ” he says. “It’s all about taking those risks, which I think not only excites me, but our audiences as well.”

Signature Theatre, 4200 Campbell Ave., Arlington; through Oct. 8, $40-$104.

Correction: An earlier version of this story gave the wrong age for Eric Schaeffer. He is 54.