A big name actor/star in a really really awful movie
Only two rules:
(a). They need to have already made it big BEFORE starring in said film.
(b). They need to have played a significant role in the film. Supporting roles still count but not cameos.
Let me start us off:
1. Adam Sandler AND Al Pacino in Jack & Jill (2011)
2. John Travolta in Battlefield Earth (2000)
3. Ben Affleck AND Jennifer Lopez in Gigli (2003)
4. Robin Williams AND John Travolta in Old Dogs (2009)
5. Robert Downey Jr in Doolitle (2020)