One of the things I discovered right away is that folks with dementia have almost no sense of humor. I say "almost" because every once in a while it will shine out through the darkness.
One night I caught a resident mimicking of one of the nurses. Funny in itself. Even funnier that that other two girls with her caught it and giggled.
I also have a resident who's been refusing to go to bed before the end of my shift. I can persuade her to let me help her into her pajamas, but she'll have no part in going to bed. Her roommate tries to mother her constantly. Last night the roommate said "You cannot make a habit of this!" The resident caught my eye and busted out the biggest grin. Ha! Oh, yes she can. And will.
On the other hand, you can't tease them. Either they don't hear you or they don't get it if they do. Better to let them take the lead and then just laugh in response. Of course, you'd better be sure they're teasing you and not being serious. Sometimes what sounds funny is not a joke to them at all.
Every day, I smile big at least once. Sometimes it's accompanied by a shake of the head, but it's a smile nonetheless.