In the critically acclaimed off-Broadway musical The Last Session, a tired and depressed songwriter who no longer wants to live with AIDS gathers his friends together for one last recording session before he offs himself, only to be interrupted by a fundamentalist Christian stand-in singer who tells the songwriter he'd better repent for his homosexuality first. Songwriter Steve Schalchlin, the former director of the National Academy of Songwriters and an HIV-positive musician himself, used his own experiences as a loose framework for the show (though unlike his protagonist, Gideon, he hadn't actually considered suicide, and his own fundamentalist parents have been supportive). What he did do, after rebounding from pneumonia with the new AIDS drugs, is create the online diary "Living in the Bonus Round," in which he recounted the ignominies of the disease and his concurrent struggles with his craft for friends and curious strangers alike. Partner Jim Brochu, a playwright, pestered Schalchlin to do something more with his stories, and wound up writing the book for The Last Session, which takes place in a '50s bomb shelter turned recording studio. Schalchlin performs the gospel and rock crossover songs from The Last Session in "Living in the Bonus Round," his one-man show beginning Wednesday at 8 p.m. (and running through May 17) at Josie's Cabaret and Juice Joint, 3583 16th St. (at Market), S.F. Admission is $10; call 861-7933. The last night of the show will coincide with the AIDS Candlelight Memorial and Mobilization, which begins Sunday at 8 p.m. a few blocks from Josie's, at Market and Castro. Marchers will proceed to U.N. Plaza at Market and Eighth Street for an outdoor celebration of the lives of people lost to AIDS, with speakers, music, and shared remembrances. Call 863-4676 for more information.