John Dowling, the afternoon voice on WMZQ, says he's done some radio acting in the past, so it was just a matter of time before he stepped into the limelight. Which is what he's doing on Tuesday and Wednesday nights at Garvin's Laugh-Inn, playing Cletis T. Fullernoy, a good ol' Texan who got hitched to the first girl who wouldn't spit on him. Cletis is one of three characters in James McLure's two-act comedy, "Lonestar." The play, once described as "a kind of male 'Vanities,' " is about three friends who drink and reminisce behind a roadside saloon. Dowling says, "The subject made it natural to tie it in to a country station."

"It's my first time in front of people, but I have no problem in front of crowds," he insists. Two summers ago Dowling was asked to keep the audience "up" between sequences during a taping of a dance concert with Julie Andrews and Rudolf Nureyev at Merriweather Post Pavilion. Which would have been fine if he'd been a stand-up comedian, which he wasn't. "But I had to stand up in front of a hostile crowd -- a full house, at that -- for 45 minutes. Once I did that I figured I could survive in front of any house," he chortles in rich FM tones.

"I love comedy, making people laugh," Dowling says. "Cletis is a very funny character, though I could probably play it a lot broader than I have. I worry about the physical business, which I don't have in radio; I'm still working on that." "Lonestar," which debuted at the Festival of New American Plays, will run through Nov. 3 and also stars Cebe Marple and Mark A. Marple. For information, call 234-7143.