George Clooney has had it with "ER," he tells PEOPLE. He says he'll no longer make any appearances on the show that launched his career, despite his special stint last May for Julianna Margulies's final episode. Then again, Clooney, 39, tells the magazine that he just might return if the Anthony Edwards character, Dr. Mark Greene, succumbs to a brain tumor. "Maybe I should come back and (escort) him off the show too," the "O Brother, Where Art Thou" star joked. "I know, I'll return every time someone wants to quit. I'll pick up alumni from 'ER' and bring them to the film community. I'll bring them to that pier where Julianna Margulies found me. I'll be saying, 'Mark, look towards the light.' "