Switch out actors in a movie and see how it changes

[CinnamonSugar]

I saw a pic on FB of a very young Kate Winslet and Michael J Fox (among other actors) and I got to wondering—

If I switch out MJF for Leonardo DeCaprio, what would “Titanic” be like?

What are some actor pairings in movie you think would be interesting?

 

[OldFeller]

I never liked the casting of Jack Nicholson in As Good As It Gets. He's great at being crass. But not belivable as a vulnerable romantic interest. He's also too old.
I've been trying to imagine someone else in the role of an OCD bigot.
John Goodman is also too old but he can play anything. I hated the Big Lebowski but Goodman showed that he could flip the switch completely believably.