When John Ritter talks about the unbridled passion between him and Markie Post on their CBS Hearts Afire series, it’s as if his lascivious Jack Tripper character from the sophomoric ’70s sitcom Three’s Company has suddenly sprung back to life.

“Some people complain, ‘Don’t they kiss too much?’ ‘Don’t they touch each other too much?’ Well, I don’t think we do nearly enough,” he says.

“I try to get as physically demanding with Markie as she’ll let me,” he lightly informs. “So far we’ve kissed in the back seat of a car, a hospital, an office building, a motel room, a bathtub, a nightclub. I guess the top of the Jefferson Memorial is next.”

If Ritter’s a trifle excessive when discussing his latest television offering, it could be partly because the sweet taste of a bona fide hit has been eluding him for a while.

Anyone who wants proof of the roller-coaster nature of show business need look no further than the ups and downs of Ritter’s career.

The actor is definitely riding high this fall. Hearts Afire has been in, or near, the top 10 shows in the weekly Nielsen ratings since its debut.

Only a few months ago, Ritter was in a gulley that seemed to get deeper with each successive project — the big-screen failures Noises Off and Stay Tuned, the quickly defunct animated show Fish Police.

“Maybe it’s TV karma or something,” he says of the latest trend in his professional life. “It was a surprise when Hearts Afire started doing so well so fast, but then, it’s always a surprise — when a lot of people come to see you, or no one comes at all.”

Hearts also enjoys a cachet of hipness as the latest product from hot producers Harry and Linda Bloodworth-Thomason, aka the TV twosome who are pals of President-elect Bill Clinton.

“I think that Linda is trying to be careful about not getting politics into the show,” Ritter says. “I know she and Hillary and Bill Clinton are very close friends and all of that, but that’s in Linda’s own life, separate from the set. I think we’ve had our one requisite Dan Quayle joke — every show has had its Dan Quayle joke — and that’s it. Besides, the series is about romance, not politics, even though it’s set in Washington.”

To hear Ritter tell it, things couldn’t get much better for him than they are on Hearts Afire. He refers to scripts by Bloodworth-Thomason as “presents,” to Bloodworth-Thomason’s written dialogue as something “that just pours into you as an actor, like good, healthy soup.” Ritter adds, “Sometimes I think I compliment Linda too much.”

As for Post, “If you could go to a computer and feed in all the information about what a perfect leading lady should be and then make it come true, you’d have Markie,” he says. “She’s fast on her feet, intelligent and selfless — I can’t think of what the show would be without her.”

Don’t expect the flames in Hearts Afire to dwindle — at least not for a while. Midway through the season’s shooting, they’re still going strong.

And why not? “We’re nowhere near rocking chairs yet,” he says.