Housed in the Ivanhoe Theater's "catacombs," whose faded brick walls partially mortared with bones date from when the Ivanhoe was a restaurant with a medieval theme and the cellar was a dungeon fun house, the Free Associates have carved a niche for themselves among Chicago's improv groups. Though they rely on the customary draw of audience interaction, soliciting viewer suggestions about characters and situations, director Mark Gagne's four-year-old company specializes in affectionate, informed parodies of popular literary works, films, and television shows. The long-running Cast on a Hot Tin Roof sends up Tennessee Williams's melodramas, while the whodunit satire Pick-A-Dick offers the audience a choice between Nancy Drew, Philip Marlowe, the Hardy Boys, and V.I. Warshawski, among others. The most recent additions are Blithering Heights, inspired by the Bronte sisters' gothic romances and the shadowy movies made from them, and the very funny late-night romp B.S., a parody of prime-time hospital soap operas (with its fluid, stylish vacuity and conflict-filled stories it's almost as appealing in its god-awful way as the real thing). All improvisers flag sometimes, and the Associates are no exception. But even when their cleverness quotient drops, the actors' fluency in the style they're spoofing, their expert portrayals of stock characters, and their skill at developing well-plotted one-acts from their fans' motley contributions makes for engaging entertainment. Ivanhoe Theater, 750 W. Wellington, 975-7171. See shows' individual listings for dates, times, and ticket prices.