As an actor, I do think about body language. Sometimes it's very effective, and sometimes not, especially in literature. Actors have the privilege of conveying emotions (or conveying the "hiding" of emotions) through body language, sometimes on its own -- it's one of the best tools an actor can have. Watch the best actors and you will notice their superb mastery of body language.
In written words, though, body language is harder to accomplish. You can only wiggle or squirm or flick your hand or nod or shrug so many times before they get really tedious. I think a little goes a long way. Most body language in literature tends to be very obvious and superficial, because without the actual visuals, it's really difficult to imagine the subtlety, unlike seeing a real person's body language.
To some extent. I find it's very easy to overdo -- the heroine in my first book kept covering her mouth with her hand when she wanted to laugh. It made sense for her as a character; she is shy and somewhat repressed. But when I did a reread of the entire book, I was like, damn, this girl never stops! I cut all but two mentions of that particular gesture.
I think it's important to keep a character's physicality in mind and to use that, but a little goes a long way.
. . . I find that level of subtext mostly unecessary in most other types of fiction writing.
Because you've got access to people's heads in a way you don't have in screenwriting. You don't necessarily need to be so coy about indicating a character's emotions when you can actually just say "he was pissed off."