[Coda]

The sensors could be in your shoulders. Plenty of people act expressive with their shoulders when they emote, so all you'd have to do is make sure that normal walking motions don't trigger it, and it would come off pretty naturally.

Of course, even easier than that would be eyebrows and ears. It would feel very natural, because smiling pushes back on the ears a little bit already, and eyebrows would just map directly.