The Doctor smiled and was looking at her, he then looked back at Jack "Good." He steps forward onto the platforms with Jack. He was interested to see how Jack did at this. He played the game with him doing his best at it, he was having fun, but he wasn't finding it easy. He though still smiled and he looked at Jack.
When the steps got a bit faster and the Doctor tired, he ended up falling over on his platform, But he was still grinning. He didn't care if he had just lost.