Even among Chicago's current hordes of talented solo performers, Marcia Wilkie is a standout. At once relaxed, unassuming, and riveting, she's living proof of the rightness of Lenny Bruce's (perhaps apocryphal) advice to young stand-ups to "throw away your act." Wilkie's act is no act at all: it's pure Marcia, alone onstage, telling the truth, unencumbered by the posing and pretentiousness that sink other better-known performers. She courageously lays her soul bare, talking freely, frankly, and quite wittily about her lesbian lover, her selfish, self-destructive sister, her bittersweet memories of being the only girl in the sixth grade attracted to other girls. What keeps her show from being just another autobiographical gush by yet another stagestruck narcissist is the way Wilkie, like all natural comedians, invites the audience to laugh at her foibles and misadventures. In fact, so tight is her bond with the audience that we often feel she's sitting right there with us, laughing away at this funny, hapless woman onstage. And it's this comic self-effacement that ultimately makes her irresistible. This show is part of the gay- and lesbian-themed Pride Performance Series. Bailiwick Repertory, Theatre Building, 1225 W. Belmont, 327-5252. Sunday, July 24, 8 PM. $12; Pride Series passes $65-$110 (call 883-1090 for information).