LOS ANGELES - It is called "blocking day" in the TV sitcom world - a seemingly chaotic ritual when the creative and technical elements of a half-hour show come together.

Consider a recent blocking day for NBC's "Stark Raving Mad," one of the season's highest-rated new shows.

The bleachers, which stretch the width of Stage 14 at Studio Center in the San Fernando Valley, are empty except for a few hangers-on. The next night, the seats will be filled with a laughing audience - or so the makers and performers of the show hope.

The stage floor in front of the bleachers is jammed with activity. Four huge film cameras, each with a three-man crew, maneuver forward and backward before the large living-room set. The actors, including stars Tony Shalhoub and Neil Patrick Harris, are having an un-Christmas party with a group of disillusioned misfits.

Just when the scene starts rolling, director Michael Lembeck calls a halt. "Camera 2, move in on Neil," he instructs. Time after time, the actors wait patiently while a camera angle is fixed.

Each time the actors resume, they are greeted with loud laughter from producers, writers and other production people, all of whom have heard the lines before. This seems to be a normal, morale-boosting reaction for sitcom makers, who hope the bleacher audience will respond the same way the next night.

And if the crowd doesn't laugh?

"Then everyone huddles," Mr. Harris says. "We come up with new ideas: `What about this? What if we scrap the whole scene and just do the first line and the last line?' We have really talented fixers on the show."

All this while the audience waits, so they need to be rather quick fixers, too.

During a lunch break, Mr. Harris sat down for an interview in an adjoining set - a bar scene. "Stark Raving Mad" is his first live- audience sitcom, and he still is getting used to the system.

"It's a very odd and fairly surreal process, much different than I had anticipated," he says. "But good, challenging. When you watch these shows, you think it's a fairly simple formula. But it is really very complex, as far as the writing, your inflection, when to wait for the joke, when to pause. It's an interesting learning process."

You would hardly recognize Mr. Harris from his days as "Doogie Howser, M.D." (1989-1993). He is 26 now, 6 feet tall, with a grown-up face and a brush of blond hair that doesn't need combing.

He has remained busy since his teen-age doc years, appearing in made- for-TV movies ("A Family Torn Apart," "Cold Sassy Tree"), features ("The Proposition," "The Next Best Thing") and theater ("Romeo and Juliet," "Rent").

He appeared with Kelsey Grammer in a concert version of "Sweeney Todd" at the Los Angeles Music Center when he was visited by "Stark Raving Mad" creator-producer Steven Levitan ("Frasier," "Just Shoot Me"), co-producer Jeffrey Richman and the pilot director, sitcom veteran James Burrows ("The Mary Tyler Moore Show," "Cheers"). They chose him to star opposite Mr. Shalhoub, formerly of "Wings."

The show's premise: a clash of two disparate characters - Mr. Harris, a phobic book editor, and Mr. Shalhoub, a manic author of best-selling horror novels.

NBC had enough confidence in "Stark Raving Mad" to bestow the cushiest of time slots, between "Frasier" and "ER" on Thursday night, and has picked up the series for a full season of 22 episodes. The show has consistently placed within Nielsen's top 15 rated shows since its debut in September.

Mr. Harris' career started at 14 when he was chosen from an actor' s workshop in his hometown of Albuquerque, N.M., to appear opposite Whoopi Goldberg in the feature "Clara's Heart."

He spent most of his teens on "Doogie Howser" and says he has no regrets.

"The series has granted me opportunities that I would never have had otherwise in my adulthood, freedoms that would have been unimaginable, financially, professionally, education-wise," he says. "That really marked my path."