For some time now, when I read to my Older Nephew, he would point at pictures of people in the books and ask, "Why is he doing that?" Now, he's gotten much more specific in his questions: "Why is he laughing?"; "Why is she looking through the window?"; "Why is he making that face?" I have no idea if this just represents additional verbal fluency, or if it's some kind of developmental indication of more nuanced understanding of other people's actions. It's interesting, though.
Older Nephew's interest in other people's mental states extends beyond books. For example, we had the following exchange while I was changing Younger Nephew's diaper. (When I say "changing Younger Nephew's diaper", picture a mixed martial-arts bout in which one competitor outweighs the other by an order of magnitude, but the smaller is armed with poo. And the ability to shriek like a banshee.)
Me: Come on, Younger Nephew, the sooner you let me change your diaper, the happier we'll both be. Older Nephew: Why will you *both* be happy? Me: He'll be happy because he doesn't like having his diaper changed. And I'll be happy, because I don't like changing diapers. Older Nephew: Why don't you like changing diapers? Me: Do *you* like changing diapers? Older Nephew: No. Me: Well then.