Nice little Jersey boy Frankie Muniz hits the big time with Fox's quirky mid-season sitcom "Malcolm in the Middle."

When 14-year-old Frankie Muniz leaves an audition, he gives his mom a signal indicating how he did — a thumb up, down, or somewhere in the middle. So last year, when he tried out for his biggest role yet, as the titular tyke on the loopy Fox mid-season comedy Malcolm in the Middle, he was sure he had a definitive answer on a potentially career-changing gig: No way, Mom.

”I actually put [my thumb] down. I just didn’t think I had it,” says Muniz. ”I thought I was too old.” Lucky for him, his usually reliable prognostication system was all thumbs. Not only did he land the part of a whiz kid with a 165 IQ, he also scored a killer, post-Simpsons slot, and even got the chance — pubescent pals take note — to say ”ass” twice in the show’s premiere.

But don’t worry about such stupendous success leading to an E! True Hollywood Story 10 years down the line. Muniz definitely knows how to keep a level head: He lives with his mother and grandfather in suburban Wood-Ridge, N.J., remains best friends with the kid up the block, and makes time for his two favorite TV shows — Once and Again and, ahem, Shasta McNasty. He’s also his own toughest critic. ”I actually went up to the director of Malcolm in the Middle after I saw the pilot and said, ‘I want to redo it!”’ says Muniz. ”I hated myself in it.”

Sounds like that downturned thumb is still on the fritz. Truth be told, Muniz shines in a part that asks him to act convincingly wise beyond his character’s 11 years, though still full of adolescent doubt and frustration. Plus, he frequently has to address the camera straight on. ”We all were saying, ‘Where are we going to find a kid who can do all this?’ ” says creator and executive producer Linwood Boomer. ”We had geared up for this gigantic nationwide search, and on the second day of casting, we got a tape from New York, and Frankie was there. It was so obvious [it would be him] right from the get-go.”

Muniz first got into acting six years ago, when he saw his older sister on stage during a production of Joseph and the Amazing Technicolor Dreamcoat. After doing regional stage work, he nabbed bit parts on everything from teen sitcoms (Sabrina, the Teenage Witch) to slightly more TV-MA material (Silk Stalkings), and has his first lead film role in the upcoming boy-meets-pooch drama My Dog Skip. But it was Malcolm that was set to be his breakthrough last fall — until Fox bumped it to January.

Now, Muniz and company stand to benefit from a hefty mid-season promotional push — a move that helped Futurama (which Malcolm will preempt for a month) last year. It may need it: Despite its delightfully subversive take on suburbia and family — and an impressively in-the-groove cast — Malcolm must buck an alarming trend. To put it mildly, this season hasn’t exactly been kind to off-kilter Fox shows (R.I.P. Action, Harsh Realm, Manchester Prep).

But even if Malcolm‘s ratings don’t live up to expectations, there may be a directing job in Muniz’s future: He plans to videotape the highlights of his wunderkind life for posterity’s sake. ”I’m meeting all these people, doing all this stuff, and I want to remember it,” he says. And if all else fails, perhaps Roger Ebert can put that well-seasoned thumb to work.

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