A Harold begins when a performer solicits a suggestion from the audience. Two or three performers start a scene, either through physical activity or conversation. The first scene may or may not have to do with the audience suggestion, but eventually the performers get to it. As the set unfolds, new scenes are started and discoveries are made, usually about people and relationships. Using what are referred to as callbacks, skilled Harold players bring earlier elements into later scenes, creating a finale that ties together the original audience suggestion and several of the characters, relationships and discoveries made along the way.

Although arguing goes against the tenets of improv, good Harold players can work it into scenes to heighten tension, create humor and even move scenes forward. ''If you're playing the same game -- 'yes and' -- you can still argue,'' Mr. Besser said during an interview while the marathon was under way on the theater's small stage. ''It's just not good to start a scene with an argument because that can end it. If the argument is funny, that's playing the game.''

Speaking of Mr. Close, with whom he studied in Chicago, Mr. Besser said: ''His long form improv makes people funnier than they are by allowing them to use the group mind.''

Ms. Poehler, who studied with Mr. Close at the Improv Olympic in Chicago, said, ''If you play the truth of things it's a lot funnier than just commenting on it.''

Ms. Poehler met Mr. Besser, Mr. Roberts and Mr. Walsh in Chicago at the Improv Olympic in 1993. The three men had been among the founders in 1990 of the Upright Citizens Brigade, which Ms. Poehler then joined. The group came to New York in 1995 and started doing improv shows. Their sketch comedy show on Comedy Central ran for three cable seasons, 1998 to 2000, and during the second year they opened their theater on West 22nd Street. The 74-seat space has shows seven nights a week and offers improv and sketch writing classes. On Sunday nights, brigade members present a long-form improv show. Performers and writers from shows like ''Saturday Night Live'' and ''Late Night With Conan O'Brien'' often sit in.

During the last night of the marathon, the Upright Citizens Brigade took the stage for the first of their three performances of the evening. Joining them were some guests: Sarah Silverman, the stand-up comic; Jon Glaser, a writer for ''Late Night With Conan O'Brien''; Mr. Sanz and Ms. Dratch from ''Saturday Night Live''; and Rachel Hamilton, a performer from Chicago's Second City troupe.