Ever since David Shepherd began experimenting with commedia dell’arte scenarios at the Compass in the late 50s, long-form improvisation, in which plays are spontaneously created before the audience, has been the Holy Grail of improv. A few local companies–most notably Mick Napier’s Pup Tent Theatre–have recently realized aspects of Shepherd’s dream. But none have come as close to creating consistently funny, literate, and well-crafted fully improvised plays as the Free Associates do in Cast on a Hot Tin Roof. Every week this plucky group of seven actors creates a new one-act parody of Tennessee Williams’s work based entirely on suggestions from its smarter-than-average audience. The night I caught the show the Free Associates created a marvelous play with the rather fetid title “Sweet, Sweet Mangoes of Flatulence” (suggested by the audience), which was, off the cuff, so well structured and on the mark that it seemed at once too intelligent to be improvised and too spontaneous to be scripted. Bop Shop, open run (1807 W. Division, 561-8718). Saturdays, 8 PM. A special “Birthday at the Bop” anniversary show Saturday, August 1, features Cast on a Hot Tin Roof at 8:30 PM, a preview of the Free Associates! upcoming game-show spoof The Hollywood Squares Not…Again! at 10 PM, and live jazz from 11 PM to 2 AM (doors open at 7 PM, and the evening also includes hors d’oeuvres and a silent auction). Regular shows: $5, “free if you look like Tennessee Williams.” August 1 “Birthday at the Bop”: $10 in advance, $15 at the door, proceeds benefit the Free Associates’ Stages Through the Ages touring educational show.