Inexperienced improvisers think improv is all a matter of speed; rushing through the preliminaries, they create slapdash characters in cliched situations. Then they wonder why they don't get laughs. Seasoned improvisers know that mental agility is more important, that time spent developing a character and allowing a scene to unfold will pay off. And no one in Chicago takes as much time or care in long-form improvisation as Robert Dassie, Rich Talarico, and Stephnie Weir, the three players in the late-night show Trio. No movement is wasted, no gesture is unimportant--everything is a possible portal of discovery. Minor accidents are incorporated into the scene, as when an audience member's suddenly beeping watch provided the sound effect for Dassie's own imaginary timepiece going off. Even more impressive is how attentively these three accomplished performers--Weir and Talarico currently appear on Second City's main stage--listen to each other, fashioning characters complex enough and dialogue interesting enough to pass for scripted. Watching Trio is like watching a master class in improv. ImprovOlympic, 3541 N. Clark, 773-880-0199. Through June 13: Sundays, 11:15 PM. $5.50.