Vi was my favorite for interactions--I think she's more approachable (and more prone to approaching random guests on her own) because she doesn't get mobbed like the movie characters do. The one time I saw Kylo Ren he was literally being followed by half a dozen people trying to proposition him.
Chewie seems to do surprisingly well at being able to move around freely, though, and I caught him and Vi (and
Chewie's handler the young man who hangs out with Chewie--never caught his name) hanging around the X-Wing chatting with guests about all the weirdly illiterate offworlders who couldn't read Aurebesh in between trying to figure out why the X-Wing kept emitting smoke. Super fun. I didn't join in that time but I had fun listening in, and one guy got a great reaction from Chewie when he "spoke" back to Chewie in Shyriiwook (aka did his best wookie impression).
I'd say just keep your eyes out and don't limit yourself to only approaching the primary characters from the movies. If you see characters hanging out on the platform with the X-Wing, it's because they're there to engage with the crowd--go up and listen in, join in the conversation if you think of something to say. Read the book so you know enough about Vi to ask about her mission--I wish I'd thought to ask her if the "rumors" were true about her founding the resistance base on Batuu when she caught me scanning a crate and set me straight on how I ought to be using my datapad to hack surveillance devices for the Resistance instead. But she showed me how to use the app and that was still a very cool interaction. Using the datapad app in general seems to be a good way to attract interactions; I saw people get stopped and questioned by stormtroopers for it and I don't think Vi would have stopped and talked to me if she hadn't caught me obviously messing with a crate.