Body language and gesture is also important in a wide variety of musical performances. Especially since music, rhythm, dance, and body movement are all so deeply intertwined. Perhaps actors and opera singers (which would also include actors in musicals... sing, act, AND dance) might manifest these traits more obviously, but body language is important to nearly all human performance.

As a musician (not a singer), I communicate with other musicians largely via body language, but the audience is largely oblivious to it. Only another musician -- and mostly just those who have played with me -- would understand the difference between lifting my bow 1 cm or 2 cm off the strings of my violin.

This is true, there certainly are many cases where the audience is oblivious. But imagine the stereotypical rock/metal band playing. Their stance, how the strut around, how they move, how their hair moves all effect how the audience reacts to the performance.

I generally count rock stars in the category of "celebrity actors", not "musicians".