Long before Clint Eastwood was urging crooks to read his lips and make his day--and long, long before Batman was redefined as a disturbed dark knight--there was fictional private eye Mike Hammer. Where Philip Marlowe and Sam Spade were downbeat loners trying to restore order to the mean streets of America, Mike Hammer was nuts--a sadistic fascist with a messianic complex in one hand and a .45 in the other. ("I lived to kill so that others could live," he growls in One Lonely Night, one of a series of novels that also includes I, the Jury and Vengeance Is Mine.) This weekend the improv comedy group Free Associates, whose specialty is spoofing literature, turn their talents to the novels of Hammer's creator, Mickey Spillane. Unlike the troupe's long-running Tennessee Williams parody Cast on a Hot Tin Roof, Bang Bang won't be constructed from audience suggestions. Instead this send-up of Spillane's lurid, commie-bashing thrillers will be improvised from a prescripted story line; but the audience is encouraged to take part in the story as it happens, moving with the actors around the Bop Shop (transformed for the occasion into a beatnik-era coffee shop). The show's a fund-raiser for the Free Associates' upcoming move from Wicker Park to Lakeview (they'll be in residence at the Ivanhoe Theater when it reopens in the fall); the price includes a cocktail weenie buffet and a 60s-retro costume contest. It should be perfect entertainment for weather like this: wet heat that hangs over the city like a huge pair of unseen pincers, clinging to the orderly piles of concrete until it's soaked with dust and debris so when it comes down it isn't rain but the sweat of the city . . . oh, never mind. Bop Shop, 1807 W. Division, 784-2745. Saturday, June 25, 7:30 PM. $15 in advance; $20 at the door.