I listen to Ken Jennings' podcast sometimes. He's said that he had to train for awhile for his appearances. Knowing the answer and being able to say it in the form of a question are only half the battle. You have to have what he calls "buzzer skills" AND you have to be able to be funny at least sometimes and you can't get nervous in front of the audience, or you won't be selected. He makes it sound more like a job interview than just being a guest on a silly quiz show.