When the folks at Sheila, who'd been improvising every Wednesday at Jimmy's Woodlawn Tap in Hyde Park, opened their north-side show a little over a year ago, they had a great gimmick--the "giant wall of plot twists"--and a lot of charm and spunk but not much else. They depended for laughs on cuteness, sheer spontaneity, and the easy absurdity of working a randomly chosen plot twist ("a sudden death" or "a love scene," for example) into an improvised sketch that had hitherto been about, say, two guys arguing over a chainsaw. But time has seasoned Sheila: their characters are stronger, more multifaceted. Edmund O'Brien has become particularly adept at creating people who would not be out of place in a serious drama. Spike Kunetz and John Pierson have evolved from a tall/short team who got easy laughs just by being onstage together to a self-assured comedy duo capable of winning big laughs with the subtlest physical bits. Even more impressive is the intelligent, confident way these six performers play with their long-form improvisations, taking the sort of risks that would trip up less experienced troupes. The night I most recently saw them perform, O'Brien was playing a kid lost for years in a mall: popping onstage for extraordinarily short blackout scenes--kid running breathlessly through mall--he was at once hilarious and essential to the main story, about a mother's decade-long halfhearted search for the difficult child. This week's show has a special Easter/Passover theme. Organic Theater Company Greenhouse, Lab Theater, 3319 N. Clark, 327-5588. Open run: Saturdays, 10:30 PM. $7.