(AP Photo/Mary Altaffer)
Sisto hums a few bars of the "Law & Order" theme, and recalls with a laugh: "There I am, walking with the others. And I thought, 'What's HE doing there?!'"
Maybe it was just time.
The 33-year-old actor has already played Jesus and Julius Caesar, a teen who fell for Alicia Silverstone in "Clueless," and, recently in "Waitress," Keri Russell's loutish husband.
Last season, he tracked down abductees on the short-lived drama "Kidnapped." And among his most memorable roles: Billy Chenowith, a gifted photographer and tormented manic-depressive on the HBO series "Six Feet Under."
Now he's joining "Law & Order" for its 18th season. He plays Detective Cyrus Lupo, new partner to Detective Ed Green (series veteran Jesse L. Martin).
"Law & Order" begins a new season after surviving a near-death experience last May. It was almost canceled, the victim of a sharp ratings drop. Having won a reprieve, it's now back in the chase to overtake "Gunsmoke" (20 years) as TV's most-enduring prime-time drama.
"It's classic 'Law & Order,' but it's been reinvigorated," said Sisto, munching a cookie at a favorite coffee hangout in his SoHo neighborhood.
The premise: After four years abroad working undercover intel, Detective Lupo is summoned to New York when his brother is found dead. Lupo joins Green to crack the case.
Unlike many of Sisto's roles, Lupo is a somewhat understated personality. Sisto has a knack for stirring up a thunderstorm in his performances. But not here.
"It's a different kind of acting, this 'Law & Order' thing," he explains. "They want two partners who can play off each other well and are fun to watch, but depth of character is not necessary in this job. Too much character gets in the way of the story. This is a very specific gig."
Sisto fell under drama's spell growing up in Chicago, where his mom, an aspiring actress, brought him along to her auditions.
"Then someone would say, 'Does your kid want to read for a part?' I did some plays and had a great time."
When he was 16, he landed his first film, the drama "Grand Canyon," playing the teenage son of Kevin Kline and Mary McDonnell.
"That got me a foot in, to come out and have a life in L.A. in this business," Sisto said. "It could very easily not have happened for me. To be an actor takes a lot of courage, but the way I did it takes less courage." He laughs.
"I was very lucky."