My DD found sample DCP interview questions online and practiced those before she had her phone interview last fall. Just answer questions honestly and be willing to work in any role they put you in. The application lets you choose preferences for roles, but my DD got QS which she didn't choose, but is fine with, she'd rather do that than custodial. They did put her in her favorite park though.. favorite park was one of the interview questions.
When to apply--pick a time you can commit to --its 4-7 mths, with specific start/end dates you can choose from. If you're rejected, you can apply again the next season. If you're going to work in the Fall, you WILL be there thru the holidays, they don't have end dates in December. So if being home for Thanksgiving/Christmas/etc is important, apply for the Spring session.. applications come out in the Fall.