Profile : Mad Matt : STAR OF FOX’S NEW SITCOM IS A BALL OF AD-LIB ENERGY

Matt Frewer had been a good boy the first time through, doing the scene the way it was written, sticking to the script of his new Fox sitcom, “Shaky Ground,” delivering the punch lines exactly on cue, just as he’d rehearsed them all week.

“Could you throw it in a blender,” he was supposed to say when his TV wife (played by Robin Riker) told him his dinner was ready. “I don’t have the energy to chew.”

The line got a laugh, as did most of the scene, but Frewer--whose bent but brilliant improvisations were the trademark of his last TV series, “Doctor, Doctor”--didn’t leave it at that. As the cameras began rolling for a second take, he started to make things up.

“Could you chew it first?” he asked as Riker watched helplessly, uncertain of what he’d do next. “And then feed it to me like a baby robin?”

At which point Frewer threw his head back and opened his mouth, a grown man waiting for worms. It was an act of unexpected lunacy that led the studio audience to do something that studio audiences rarely do. They laughed at the scene for a second time, even louder than they did before.

“I just like the excitement of it,” Frewer would say later, asked why he wanders so often into unscripted territory. “I like the idea of taking a step into the unknown. I mean, I don’t think you become an actor for security. You become an actor for the danger.”

Which is why, ever since his days as America’s first computer-generated pop icon, the latex-laden “Max Headroom,” Frewer has developed a reputation for veering away from his scripts into all manner of unplanned weirdness. And though “Shaky Ground”--in which Frewer plays a frazzled white-collar worker trying desperately to balance the pressures of a dead-end job and raising his three-kid family--may seem like just another mainstream sitcom, Frewer still manages to turn it into something fairly strange.

All night, Frewer’s brain reels into a free-association free fall. Coming up with descriptions for his in-laws’ flatulent old dog--a key character in this episode’s story--Frewer’s ad libs go from merely twisted (“He smells worse than John McEnroe’s inner soles”) to borderline incoherence (“There’s the little Minoxidil dip sausage.”)

“On the night of a show,” Frewer says, “my brain is working about 80 times as fast as it does normally. So a lot of stuff that happens surprises even me.”

To watch him work like this, mugging and whining and occasionally dancing around the sound stage like some kind of deranged stork, you’d never guess that Matt Frewer was trained as a Shakespearean actor. He grew up in Ontario, Canada, planing to be a professional hockey player. The pro scouts were already hot on his trail when he suffered a career-ending thigh injury at age 15 and, despite his parents’ objections, he went to acting to pursue a theatrical career.

After three seasons with the National Youth Theatre of Great Britain, he was accepted at the prestigious Bristol Old Vic Theatre School where, he says, he learned the discipline that enables him to take so many chances now.

“You need that technique to fall back on” he says, sounding mighty serious for a man who’s been known to take his pre-show introductory bows with a balled-up T-shirt stuffed in the crotch of his pants. “I think to have classical training is very important, to have studied movement and voice. That’s what allows you to go off and do things.”

Frewer was working in Shakespearean repertory companies in England when, in 1984, he got the part of Max Headroom. What started as gimmick for a music video show in London quickly became England’s hottest talk program. By 1986, Cinemax had picked up “The Max Headroom Show” and Coca-Cola was using him as a spokesman. In 1987, ABC ran a dramatic series featuring the character (in which Frewer also played Max’s human alter ego, TV reporter Edison Carter). Max had become a worldwide celebrity, but Frewer remained virtually anonymous. Even now, he says, lots of people don’t realize there was a real human being behind the rubber makeup.

“I got jealous of the attention Max was getting, I do admit that,” Frewer says. “That was weird for a while.”

But, more importantly, the character got Frewer back across the Atlantic. “I always knew I’d have to go to the States at some point. North American actors rarely get a crack at anything good in England. You inevitably end up playing roles like ‘second soldier from the left.’ ”

Frewer had several feature film parts before “Doctor, Doctor” started airing in 1989. The show, beloved by critics and an adoring cult audience, was a perfect vehicle for Frewer’s gonzo improvisations, but in the beginning Frewer didn’t cope all that well with the pressures of weekly TV.

“I used to get so wound up before every show that I’d throw up. I was this sort of bulemic comedian,” says Frewer, who, thankfully, doesn’t have this problem anymore. “It was the result of this sort of self-induced comedic frenzy. I probably shouldn’t say self-induced, should I? Makes it sound like I shoved my finger down my throat.”

Even though he’s back on TV, Frewer wants to keep working in feature films. He’s in a quirky independent feature called “Twenty Bucks” due out next spring and there are tentative plans for a Max Headroom feature film. But whatever comes along, Frewer says he’s ready, that he’s always willing to take a chance.

“There are always two little men inside you, one saying, ‘Don’t do it,’ the other saying, ‘Do it,’ ” Frewer says, asked if he ever feels an inclination to play things safe. “I’ll always listen to ‘Do it.” I’ll always take the plunge, whether I’m scared or not.”

“Shaky Ground” airs Sunday at 7:30 p.m. on Fox.