When Johnny Carson comes out to do his opening monologue on "The Tonight Show" he stands on a star laminated into the floor. When Pat Sajak comes out on the new "Pat Sajak Show" on CBS, he stands on a question mark.

"We don't know yet. We're trying to low-key everything," Sajak explains in his shiny new office. "We may move on to other punctuation as the show progresses. We may go to a semicolon."Host for six years of both the daytime and nighttime versions of the smash-hit game show "Wheel of Fortune," Sajak will remain with the nighttime edition, not as career insurance, he says, but because he has a contract that runs another year and a half. Of course if he really wanted to get out of it, he could have. Sajak seems a man smart enough to know when to play it safe.

Unassuming, self-deprecating, mild-mannered and boyish, the 42-year-old Sajak may be an odd choice to steal thunder away from anybody much less the great Carson, who has a quarter century of late-night dominance under his belt. Sajak insists, as all Carson challengers have insisted, that the point is not to unseat the mighty Johnny but merely to improve upon the lousy ratings CBS gets in the time period.

Sajak's 90-minute show, which premiered Monday, replaces a bunch of cop shows and reruns which have been lumbering along for months. "And the economics are such," says Sajak, "that if we did the same rating that they're doing now with their cop show, the network would make millions more a year because it costs less to produce this thing.

"They have not verbalized this to me," Sajak says, "but I would be very surprised if in some dark recess of this building, someone hasn't sat down and said, `You know, it might be two years or three years or five years, but Johnny's going to leave eventually, and if we're in place with a show, we'll be in a nice position."'

Sajak does not apologize that his show isn't in any noticeable way revolutionary. The new set, which cost CBS $4 million to build at its Television City facilities, consists of a desk, a sofa, a chair, a place for the musicians, and seats for the audience. The band leader will wear funny clothes. "When people ask about these revolutionary changes, I'm looking for suggestions," Sajak says. "Do we do it in Latin? What exactly do we do here? Talk, in a basic form, seems to work."

Much of the set is beige. The couch is beige, the desk is beige, even the host seems beige. It may help to be beige in this job. "Oooh, uhh, well, to some extent that is probably true," Sajak says. "I don't take beige to mean bland, but inoffensive?" He nods yes.

Behind the desk, artists have painted a backdrop that is supposed to be the view from the roof of Television City. Sajak points to a tiny billboard on the backdrop. "It was advertising Geraldo Rivera, complete with bandaged nose," he says. "So we had them sort of paint over that."

He doesn't feel his show is affected by the TV trend toward tabloid trash. "No, I feel repelled and disgusted by it," says the nice young man from Chicago, Ill.

You can't talk to Pat Sajak without asking about Vanna White, that lovely letter-turning gal on "Wheel of Fortune." Sajak seems always to have been charitable and encouraging about White's huge success and yes, he will have her as a guest on his show-the Jan. 23rd edition. "It's the same show on which Barry Goldwater is booked, so we think that's a good pairing," he says with a grin.

Asked for his impression of "Goddess of Love," the howlably awful NBC movie in which White starred, Sajak says, "It seemed like about two hours long." Then he gets more candid. "For what it was, it was pretty bad. You know, I told Vanna, with that script and that dialogue, Helen Hayes could not have made anything out of it.

"It was the dippiest film! And I felt bad for Vanna, because she does have, say what you will, a nice presence to her and given a nice light Judy Hollidayish role, I think she could pull it off. But to try to do that kind of stuff was just murder for her. I blame whoever talked her into doing it."

Sajak has a two-year contract with CBS at a reported $60,000 a week, and even if CBS kills the show after 13 weeks, he still gets paid the whole kaboodle. A lot is riding on this venture, the first time a network has tried to oppose Carson with its own talk show in 17 years. Yet on the eve of the premiere, Sajak seemed relaxed and sane.