Just curious

I watched this yesterday, very enjoyable too. However, one bit really caught my attention and I had to go back after it finished to take another look. It is the scene where Prinny ( Peter Ustinov) is playing the harp accompanied by Brummell, however it was Lord Mercer (James Donald ) that caught my eye, he is clearly finding something very funny and is almost at the point of corpsing, yes, they are meant to be laughing at the song Prinny is singing, but, I suspect there may have been something else that made James Donald laugh, he certainly looks different to the staid dour men he often played and it was quite charming to see this side of him.