Great gif! He has such a wide, crooked smile that is totally infectious.
And in that scene his smile conveys so much of his sassy, overly confident attitude that drives the character at that point. BC is in such control of his facial expressions (and his body movements too) that he doesn't have to say a word to get a million characterisations across in a fraction of a moment of time.
Thanks for posting this!
"You're going into the water... short-arse!" - Sherlock