''He's always been full of shtick.'' That's how Jason Alexander's mom describes her baby boy. And the facts bear her out: At 6, he was doing Bill Cosby in his Maplewood, N.J., schoolyard. ''It was a preemptive strike against being the ridiculed fat kid,'' Alexander says. At 17, he was doing William Shatner at Boston University. ''I thought that being a good actor was being a good Shatner.'' At 29, he did Woody Allen on his audition tape for Seinfeld. ''I went out, bought glasses, and worked on the halting, breathy delivery.'' It all worked: Cosby won him friends, Shatner snagged him a scholarship to B.U.'s drama program, and Allen got him a six-year (so far) run on the No. 1 sitcom as George Costanza, lord of the losers.
At 36, who's Alexander doing next? Dick Van Dyke. Stepping into the shoes of the long, leggy, acrobatic actor in ABC's remake of the musical Bye Bye Birdie (Van Dyke starred in the 1963 film version) might seem Alexander's biggest stretch yet. But the truth is, this is the real Alexander. Most Seinfeld fans aren't aware that he's a veteran Broadway song-and-dance man, and he does plenty of both in Birdie. His character, rock-star manager Albert Peterson, shakes off his overbearing mom and turns his life around in the time it would take George Costanza to find a parking space. And he gets to romance a former Miss America (Vanessa Williams).
''It's the most amazing leap,'' says Alexander of his leading-man role. But Williams says there's a Gene Kelly soul inside that tax-accountant body. ''He just lights up as soon as he starts to sing or dance,'' she says. ''He has a very sexy side that women can certainly get turned on to.'' (Those roses he sent to her trailer the first day probably didn't hurt either.)
Alexander admits there's one kind of woman that always goes for him: mothers, ''particularly those with daughters they'd like to see in a relationship. They come up to me and say I'm adorable I'm so much handsomer, and thinner, and I have more hair.'' TV, he adds with an ironic smile, ''is apparently just not kind to me at all.''
Which may explain why he works nonstop to prove his range. In addition to Seinfeld, he records the title character's voice in USA Network's animated series Duckman, shills pretzels for Rold Gold, hosted the Emmys this year, and in January stars in Fox's big-screen comedy Dunston Checks In, about a boy (he plays the dad) and a jewel-thieving orangutan. Jerry Seinfeld picks Alexander's work ethic as one of his traits at odds with the indolent George. ''He's 150 percent game for anything,'' says Seinfeld. ''If the scene calls for him to run out of the bathroom with his pants around his ankles and then dive over the table behind the couch, he'll do it, no problem.''
This afternon, while Alexander waits in his dressing room for a Seinfeld run-through, his jeans are firmly in place. The room is bare except for a portable CD player, a few CDs (the Manhattan Transfer, Patti LuPone), and, for his son Gabriel, 3 1/2, a red Barrel of Monkeys game. He proudly displays a photo of his wife, Daena Title, a writer and ''my best friend.'' They met 16 years ago in New York while he was working at a casting agency. She was an actress then, and he put her through an entire phony audition before mustering the courage to ask her out. They married two years later, and now he makes a concerted effort to get home every day by six. (The couple is expecting a second son in February.)
''Husband, father, mensch'' that's the line Alexander's mother, Ruth Greenspan, a retired nurse, uses to describe her son. She can't help mentioning the house and car he bought for her and his dad, Alex, a retired manufacturing-plant manager. ''He's pretty much been an adult all his life,'' she says, partly because his half siblings, Karen and Michael, were so much older (by 12 and 20 years, respectively). ''At five he told me he wanted to be a presenter. 'What's a presenter?' I asked. 'You know, Ma, a person who says, Ladies and gentlemen, I now present...'''
Growing up in Jersey, Alexander studied singing and dancing, worked in children's theater and dinner theater, and (he swears) attended about 40 performances of the musical Pippin. The summer after his junior year at B.U., he was cast in the low-budget 1981 slasher movie The Burning. He then left college for New York City, where he roomed with friend and Burning colleague Holly Hunter. (Along the way, he found his real name, Jay Scott Greenspan, too long for a marquee, so he combined Jason from his mother's expression ''Jay my son'' with his father's first name.) He won a Tony for his performance as the narrator a presenter in Jerome Robbins' Broadway in 1989, the same year he got his breakthrough parts as the slimeball lawyer who tries to rape Julia Roberts in Pretty Woman and as George in Seinfeld.
What makes him so right as George? They share a certain neurotic insecurity. Says Seinfeld, ''At any moment [Jason] thinks he could be brutally attacked.''
Alexander doesn't disagree. ''As the 2000 Year Old Man, Mel Brooks said that everything comes out of fear. I think he was right. The kid who was always scared of getting picked on is still a very large part of me.''
Remember, this is the kid who took tap-dancing lessons four towns away so he wouldn't be recognized (''I was the only guy in the entire dancing school''), who began losing his hair at 17 (''I cried for a year''), and who proposed to his wife (who is three inches taller than he) four times until she accepted the offer after it flashed on a Times Square billboard.
Whether Seinfeld returns for another year is up in the air, but Alexander is always ready for the other shoe to drop. ''We like the show so much we don't want it to end when it's already old news,'' he says. ''Right now we're doing great, the shows still have spark, but the talk of the town is Friends and ER, and that used to be our slot.''
Alexander's star turn in Birdie is one sure sign that there's life after Seinfeld, but don't expect the folks to relax. Just ask Mom. ''He's instilled us with 'Will I ever work again?''' she says, laughing. ''Even when he became a professional, I thought, 'He's gonna starve. I'm in Florida. Who'll make him a sandwich?'''