Sequence by Mood
You might want to think twice before placing that photo of someone popping Molly next to one of someone reading a book (kinda awkward), but maybe that's what you're going for.
If you are keen on creating a mood with your work or telling a story via mood-driven "vignettes" or scenes, group similar emotions and facial expressions together. Then think about the order in which you want to evoke these different emotions—what journey would you like your viewer to take?
You can move back and forth between moods and make this as complex as you want. It often ends up that way anyway when considering other sequencing factors.
Just like with color, being gradual will help you with transitions. Take three photos—a teethy grin, a closed mouth smile, and a more serious face. You can start with either the teethy grin or serious face, depending on what you want the first emotional impact to be—and then put the closed mouth smile in the middle to create a nice progression.