Ron Leibman, known for his role as Dr. Leonard Green in "Friends," has passed away at the age of 82. The stage actor, who was in dozens of films and television shows, died on Dec. 6 in New York. His agent, Robert Attermann, confirmed the actor's death but did not elaborate with any details. However, a source close to Ron Leibman told several outlets the cause of death was pneumonia, according to The Washington Post. He is survived by his wife, actress Jessica Walter.