Complex sentence: Actors Alec Baldwin, Harrison Ford, Ben Affleck, and Chris Pine have played Clancy's most famous fictional character, Jack Ryan; his second-most famous character, John Clark, has been played by actors Willem Dafoe and Liev Schreiber.
Split into simpler sentences: Actors Alec Baldwin, Harrison Ford, Ben Affleck, and Chris Pine have played Clancy's most famous fictional character, Jack Ryan. Another well-known character of his, John Clark, has been portrayed by actors Willem Dafoe and Liev Schreiber.