So you didn't get the part . . . you wanted . . .
Young Actors: when you get cast--but not in the role you wanted:
A. YOU GOT CAST, CONGRATULATIONS!
B. Suck it up, and either do the role or do zero role in that show. (It's just that easy).
I get it. You thought you were perfect for the role you sought. You did the work. You see yourself in that role. You don't see the other people who auditioned for that role in YOUR role. Well, here's the skinny: At least you plus one other person casting has to see it that way. The role you wanted--despite your great desire to do it--was never yours in the first place. And that's ultimately okay. It doesn't mean it will never be a role you do. It just means you're not doing the role right now. And you'll live.
But you HAVE been cast. You can do that role. Or you can not do that role. It's a simple choice. I recommend doing the role. I recommend figuring out the value of that character to the world of the show, and giving it the same gusto you had planned for the role you desired. First off, it's just more satisfying when you create a fully formed character. Second, guess who's directing you or watching you? The guy that may or may not have seen your full potential in the audition. You now have another chance of showing them a broader picture of the skill set you may or may not have adequately displayed in your 5 minute audition. Your first show with a director is your extended callback. So why not shine as 3rd tree from the left--so they can imagine you shining in larger roles.
So suck it up, buttercup. You've been cast. And there's a wastebasket full of individuals who would kill to be in your disappointed shoes. Make the most of this opportunity to grow in your craft--and to better showcase your talents to the folks doing the casting. You've been given a gift. Say thank you, and enjoy it.