Sorry about how short this post is, I do not have much time to write it.
People do not always act the same. We each have our own ticks. Body
language can make your characters come to life in more ways than one may
think because it often tells us something the character does not want
us to know.
In Scarlet by Marissa Meyer, the character Wolf is never able to sit
still. He bounces his knee or drums his fingers. He is all in all an
energetic (and nervous) person. To say that he was not moving at all
would reveal either fear or peace in him depending on the circumstances.
In my book, Bonded Souls, Lan has this way of showing her nervousness
even when she doesn't want to. She will bite her lip or knot her skirt
in her fist. Any time I want to show this emotion in her, I only have to
describe one of these things and it becomes obvious.
Ticks can be helpful but also harmful. Remember to use them
sparingly. Not every character in your novel should have one. Those that
do should also not have theirs mentioned in every scene. It would
become repetitive and contrived in the reader's mind.