The cast doesn't 'stagedoor' before the show. However, if you catch one of them heading in or hanging out outsider (and they're not rushing or on the phone or something), you can approach them and talk to them. Since Johnny rarely stagedoors, before the show is actually your better chance of seeing him.
If there's a matinee before the show you're seeing, then the cast will be stagedooring after that show so you can catch them that way, too.