The directorJudd Apatow has temporarily returned to his stand-up roots. Mr. Apatow, the writer and director of ''Knocked Up'' and ''The 40-Year-Old Virgin'' and the producer of ''Superbad'' and the forthcoming ''Pineapple Express,'' was a stand-up comedian for seven years before moving behind the camera in 1992. (He gave up performing, he said in an interview last week, because ''I lacked something very specific: charisma.'') He has taken to the comedy-club stage again, both in Los Angeles and at the annual Just for Laughs festival this month in Montreal, where he headlined a stand-up show, as well as performing unannounced at other festival shows around town. The main reason for his return, he said, is that his next film, ''Funny People,'' to star Adam Sandler, is about comedians, and he wanted both to ''remind myself of the terror of doing stand-up'' and to develop material for the comics in the film. [ABSTRACT FROM PUBLISHER]