One common word is impersonator:
[Merriam-Webster, from impersonate]
: to assume or act the character of : PERSONATE
A more recent term is cosplayer:
[Wikipedia, from cosplay]
Cosplay (コスプレ kosupure), a portmanteau of the words costume play, is a performance art in which participants called cosplayers wear costumes and fashion accessories to represent a specific character.
The kid saw a Han Solo impersonator at the new Star Wars theme park in Disney world.
The kid saw a Han Solo cosplayer at the new Star Wars theme park in Disney world.
I get the impression that an impersonator more generally impersonates a real person than a fictional character, but the dictionary definition doesn't support that, and it may just be how I've come to use that word myself.