If you and your opponents already have some of the characters, the game will give you the option to choose between Crown and Challenge when the bar gets completed or when you get a crown in the wheel. If you choose the crown, you will have to answer a question correctly in order to get the character. If you choose the challenge, you will have to select the character for which you want to play (as long as your opponent already has such character) and you will both have to answer six questions. If you answer more questions correctly than your opponent, you will get their character. You will lose your character if your opponent wins the duel.
Bear in mind that if you are invited to a challenge during a classic game, you will not be able to reject it.