Letum's suggestion to watch expressions in a film worked for some ASD college acquaintances and me. For them, it was anime or comedy movies. They watched lighthearted shows to escape from depressing or dangerous family situations, and learned 'good' emotions that way, since there was little happiness expressed for them to imprint on.
For me, it was realizing that other people watching movies would smile at certain parts, or furrow their brows, or open their mouth at certain times. I was puzzled at first, thinking "Why is this one person miming the actor?" Then I noticed that most people watching movies do the same thing -- "Why are they all doing it? It looks silly!"
Much later, my friend who watches anime for his 'happiness fix' explained that he is living vicariously through the characters in the shows he watches -- he says you can't get the full impact of the show without feeling what the characters feel. I can understand that to some degree, like I can understand that I'm missing the words when the sound is off. I still don't 'get' many subtleties that people are trying to convey, even after reading loads of books and writing college research papers on non-verbal cues.
I hope what I said makes sense. Best of luck with learning about expressions!