Complex sentence: Ben Affleck, who was a relative unknown at the time except for appearing in "Dazed and Confused", was cast as Shannon Hamilton, he later achieved fame and starred in films such as Good Will Hunting and The Sum of All Fears.
Split into simpler sentences: Ben Affleck, who was a relative unknown at the time except for appearing in "Dazed and Confused", was cast as Shannon Hamilton. He later achieved fame and starred in films such as Good Will Hunting and The Sum of All Fears.