Final answer:
John Ritter's last film was 'Bad Santa,' where he played a mall manager dealing with Billy Bob Thornton's troublesome Santa character.
Step-by-step explanation:
The last film appearance of John Ritter before his untimely death was in the movie Bad Santa. In this dark comedy, Ritter played the role of a mall manager who has to deal with the inebriated and troublesome character played by Billy Bob Thornton. The film has gained a cult following for its unorthodox take on holiday movies and Thornton's performance as the vulgar, but somehow endearing, Santa impersonator.