They were required to play chess for the last scene of the movie. It was supposed to be one of the simplest but it ended up being one of the most problematic. It went something like this, the director Bryan Singer set up the chess table for them and asked them to play. Ian (Magneto) and Patrick (Xavier) just looked at each other and admitted they didn’t know how to play.
The director just rolled his eyes and he brought someone to teach them. To their surprise it was a Canadian grand master! He taught Ian and Patrick the basic moves and ran them through the game rules. The chess player stayed all day and was excited to be on a movie set for the first time.
Bryan Singer couldn’t believe he had hired two actors that didn’t know how to play chess. At the end, Xavier wins the game. You can see the scene here