I don't think what they say ("I want to win a NOOOOOOOOOOOOO CAAAAAAAAAAAAAAAHR!") is as important as how they say it. You want a contestant who is cheerful, fairly outgoing, and who will register delight when they win prizes and won't be a sore loser if they don't, and who will register surprise at, say, the prospect of playing Plinko. They must also not come across as needy or desperate, or disoriented as some of the Las Vegas contestants chosen at random with no human intervention do. The thing to remember is that it is theater, and job #1 is to entertain the home audience by having contestants who are likeable.