Sooner or later, the show business gods are going to whisk actress/singer Debra Tidwell out of Washington for the big time, and we'll all sit around, fondly recalling the good old days when she was ours alone. In preparation for that eventuality, you can stock up on memories at the Harlequin Dinner Theatre, where Tidwell is headlining that infectious tribute to the music of Fats Waller, "Ain't Misbehavin'."

Actually, Tidwell is very much part of a group endeavor at the Harlequin, where five singers and seven musicians are stirring up two hours' worth of classy sass with such numbers as "Honeysuckle Rose," " 'T Ain't Nobody's Biz-ness," "The Joint Is Jumpin' " and "A Sin to Tell a Lie." But you're still going to notice Tidwell first and foremost.

She possesses one of those remarkable voices that are by turns back-room bluesy and musical comedy upbeat, dusky as wood smoke and clear as a morning trumpet. The elegance of a 1940s supper club is hers, but so is the spunk of a marching football band. She can toss off a sophisticated lyric as if it were a mink coat among the many in her possession. And then she'll turn around and wrap herself up in another lyric so tightly and so intensely that you'd swear the words were keeping her warm. "Ain't Misbehavin' " lets her range from A (anguished) to Z (zestful), with stops all along the alphabet.

The show itself is eminently suited to the confines of dinner theater. More in the nature of a cabaret, it doesn't require scaling down, as most Broadway musicals do, and the performers can beam their efforts boldly and forthrightly into the laps of the spectators. Its only business is celebrating the many moods of Fats Waller, his mischief, his humor and the sad pain that frequently inhabits the souls of seemingly jolly entertainers. With a minimum of the sort of adulatory fuss that can sometimes charactertize these outings, it starts right out singing, and doesn't stop until it has run through more than 30 numbers associated with him.

Not everyone in the Harlequin cast is of Tidwell's caliber, but some of them are giving her a good run for her flash: the ebulliently flirtatious TiaJuana Rountree, for example, who makes "Squeeze Me" a delicious exhibition of cooing; and the long-limbed Ronald White, who is the dancer of the bunch and in his moments of abandon suggests nothing so much as a jitterbugging spider. Valerie Scott's stage personality is somewhat blander, although she manages nicely with "Keepin' Out of Mischief." Tyrone Aiken's good-natured roly-polyness is just what is called for in ditties like "Fat and Greasy" and "Your Feet's Too Big." All he has to do now is let himself go with his instincts.

The show regularly shuffles up the performers, who move from solos to duets to trios to quintets so briskly that the occasional weak moments are never allowed to grow into minutes. Any individual deficiencies are more than repaired by the group effort. (Even if all else were to fail, which is clearly not about to happen, there's musical director Thomas Tumulty, coaxing a virtual tornado from the ivories.) When the five singers, imitating musical instruments, join forces with the band for a session of jamming in the second act, "Ain't Misbehavin' " is close to pure party time.

The compelling reason, however, to catch this energetic little revival, which runs through Nov. 18, is Tidwell. I advise you to act now. That way, you'll be able to tell everyone you knew her when.