you probably don't have to worry about big casts these days, and you only have to worry about canon updating if you choose a canonpoint before the ending (but if you want to do that then i guess it works like any other canon update? unless you're picking a particularly weird canonpoint).

in general, remember that john isn't the dumb innocent nerd baby fandom likes to flanderize him into. he makes stupid choices based on not really thinking things through, but he's also a pretty smart guy (at least in terms of book smarts) who figured out the alchemy system mostly on his own. he's a nice enough guy, but he's not the most considerate and he can be kind of selfish sometimes in that "suburban middle-class kid who's dad probably spoiled him" kinda way. otherwise, his characterization will depend a lot on your canonpoiint since he does have some character development.

it'll help to just go back through his pesterlogs to get reminders of how he talks, but i'm also a fan of this little graphic someone made, it might help you with getting a feel for his word choice as opposed to the other kids:

http://pointlesshomestuckstatistics.tumblr.com/post/155604547638/theorigamiphoenix-a-tldr-john-and-jade-talk