Jamie-Lynn Sigler is not a Mafia princess. She's not even Italian. But the instant you meet her, you're on your best behavior. You sit up straight. You call her "Ms. Sigler" and nod a lot, politely. You accord her the deference any sane man grants the daughter of a homicidal mobster.

Because, at least initially, she is Meadow Soprano, the bridge-and-tunnel Ivy Leaguer who has pouted and connived through three seasons of HBO's Cosa Nostra drama, "The Sopranos." In her own way, Meadow is the most dangerous character on the show, since she has her father -- family patriarch and goombah-in-chief Tony Soprano -- wound tighter than a garbage-bag twist-tie, which makes her far more menacing than she looks.

Not that you're looking. No siree, you certainly are not looking, not sitting in the restaurant at the Marriott in downtown Washington on Saturday afternoon. The characters on "The Sopranos" seem so compellingly alive that even here you worry that Tony himself is about to lay a big, disappointed paw on your shoulder. All this, despite Sigler's protests that she and Meadow aren't very much alike.

"Meadow is introverted, kind of somber, not over-enthusiastic about anything," she says, cherub-cheeked and smiling. "Me, I'm a happy-go-lucky 20-year-old, very optimistic, very close to my parents."

Distinguishing the actress from the role is something of a mission for Sigler. A few weeks ago she released her debut album, "Here to Heaven," a lighter-than-air jumble of teen-pop, ballads and Spanish-language numbers, and she has come to Washington for a one-shot Saturday night appearance at Platinum nightclub. Singing is her original passion; she tried out for "The Sopranos" because she thought it was a musical show.

But as she promotes "Heaven," she's discovering a problem: Some people don't want to meet Jamie-Lynn Sigler. "I've had people who refuse to call me Jamie," she says. "They call me for interviews and they're like, 'Is this Meadow?' I'm like, 'Uh, it's the girl that plays Meadow.' I go and do radio interviews and I'm more than willing to talk about 'The Sopranos' as long as we talk about the music, too, and there have been instances where we do a radio interview and they call me Meadow the whole time. I don't want to be rude, but let that go for at least 20 minutes, my gosh."

There are, it turns out, plenty of differences between Meadow and the Long Island native who plays her. Jamie-Lynn talks faster than Meadow, and she has none of Meadow's spoiled, disaffected manners. Jamie-Lynn is cheerful and warm, half Cuban and half Jewish, and has been singing and performing in community musical theater for years. She was touring the country as far back as seventh grade. "I studied for my bat mitzvah while traveling with the road show of 'It's a Wonderful Life,' " she says.

Jamie-Lynn has enough business sense to buy a house in Las Vegas, a place she likes not just for its dry weather and shopping but its tax advantages. ("What I save in taxes alone by becoming a resident there will pay for the place.") She thinks Billy Joel is the greatest singer ever, though she's fond of rappers like Jay-Z. She broke up with her last boyfriend, who is very much alive, which is more than can be said for Meadow's last love, who is very much dead.

"I've been single for a year and a half now," she sighs. "It's tough. I finished 'Sopranos' and went on the road tour for 'Cinderella' and every guy in the cast was gay, so there was no relationship there. Then I got back and finished this album in three weeks. Now the show is filming again."

The fourth season of "The Sopranos" started production last week, though Sigler is sworn to secrecy about the plot and couldn't say much about it even if she wanted to tempt a lawsuit from the show's secrecy-obsessed producers. The scripts are handed out to the actors one week at a time, so as of Saturday she'd only laid eyes on dialogue for the season opener.

Work on the show starts at 5 a.m., with most of the time spent rearranging cameras and rehearsing. "All my friends want to come and hang out on the set, and I'm like, 'Trust me, you're going to sleep a lot.' It's hurry up and wait, hurry up and wait." The highlight is usually when production halts for the arrival of the Mister Softee ice cream truck.

For "Soprano"-philes, Sigler is happy to dish about her colleagues. Tony Sirico, the guy who plays Paulie Walnuts? "What you see is what you get. The character is basically Tony." Edie Falco, who plays Carmela, Meadow's mom? "She's nothing like Carmela. She's very low-maintenance." James Gandolfini is intense, very paternal and buys everyone dinner every Friday after shooting is completed. Robert Iler, who plays Meadow's brother, has been virtually a shut-in since his summer arrest for marijuana possession and robbery.

"It's been really hard for him," Sigler says. "It's hard enough to deal with something like this, but to have the whole country know about it makes it even more awful."

"The Sopranos" is slated for only five seasons, so Sigler's search for life after Meadow is already in full gear. Hence the show at Platinum, which turns out to be something of an anticlimax, in part because the crowd of college-age kids are a little old for Britneyesque material, and in part because it's only two songs long, with a couple of backup singers who seem, at moments, to be winging it.

Her couture is Paramus chic -- black T-shirt with the Rolling Stones tongue-lick logo, jeans and sneakers. Her dance moves tend toward the flirty with plenty of sassy tsk-tsking aimed at overeager lovers. She sings "Bada Bing," which gets its name from the "Sopranos" strip club, then "Cry Baby," her first single. She exits by high-fiving some audience members and is gone a mere 10 minutes after she arrived.

Sigler isn't fretting much about the album's sales, or radio spins. If "Heaven" flops, she'll find something else, either in the theater or in films or maybe another television show. Or she'll head back to college. She has taken a leave of absence from New York University, and she'd like eventually to earn her undergraduate degree in psychology.

"When this stops being fun," she says of the entertainment biz generally, "I'm out."

Whatever you say, Ms. Sigler.

Seriously. Whatever you say.

Jamie-Lynn "Meadow" Sigler, who has been singing in musical theater for years, goes for the high notes Saturday at Platinum.Sigler, who recently released her debut CD, belts it out at Platinum.