Favourite reactions in film?

I'm rewatching Silver Linings Playbook again right now and Bradley Coopers reaction to Jlaw saying he thinks she's crazier than he is in that cafe scene is absolutely fucking hilarious. His eyes widen and he makes his jaw all rigid like 'well fucking obviously' it's such a minor but perfectly acted moment.

Name some other reactionary moments in movies that are super minor in the grand context of the movie but are still nonetheless living rent free in ya heads.