Paul Rivers is a fictional character from the film "21 Grams," directed by Alejandro González Iñárritu. He is portrayed by actor Sean Penn and is a college professor who struggles with the consequences of a tragic accident that intertwines his life with two other characters. In the film, Paul is depicted as a man facing a terminal illness, which adds depth to his character as he grapples with themes of mortality and redemption. His journey explores the impact of loss and the search for meaning in life, making him a complex and relatable figure in the narrative.