4. DUE DATE
These days Sheen is perhaps best known as the star of TV sitcom Two and a Half Men, in which he plays a more sedate version of himself (a disgustingly wealthy hedonist with commitment issues). Charlie Harper, his character from the show, also ended up making a cameo appearance, along with TV brother Jon Cryer, towards the end of the Robert Downey Jr/Zach Galifianakis road-trip comedy Due Date.
Weirdly, even this cameo contains references to drugs.
Watch the scene here.