To know Kirstie Alley, you first gotta know something about Sharon Stone. In March, Stone told Barbara Walters that she was tricked into flashing the audience in Basic Instinct. In April, Stone said that she didn't mean to pose topless, cupping her breasts in her hands, for Vanity Fair. Please, we thought, make her stop. Then came Kirstie to the rescue. On Late Night With David Letterman in May, Alley said now that her six-season stint on Cheers had ended, she planned to become ''a spiritual adviser for wayward actresses who just don't know what's going on.'' In a tripping, bitchy riff on Stone, Alley offered this bit of wisdom: ''When that camera is in there beelining in on your V, you're gonna get a picture of that.'' Good advice, and vintage Alley. In an age when many stars won't fire off a word without a publicist's approval, Alley, 38, is a loose cannon plated in brass. This is the woman who, upon accepting her '91 Emmy, thanked husband Parker Stevenson for ''giving me the big one.'' Even her own work isn't spared her tongue. She has high hopes for the third of her Look Who's Talking movies (Look Who's Talking Now, coming in November) because ''it reminds me of the first one. The second one was lame.'' Growing up in Wichita, Kans., she says, she learned from her father to ''tell the truth, and you'll always be happy.'' The philosophy is not without peril: ''Sharon Stone is gonna kill me someday,'' she says. ''She's just gonna rip my head off.'' But until then, at least Alley will never be boring on talk shows. ''Actors start believing their own publicity,'' she says. ''Like I will believe this. I'm cool. It says it right there.''