Does your protagonist have a nervous tick? Playing with their hair, biting their nails, fidgeting, etc.?
When Citrine is nervous, she tends to crack her knuckles. Once that’s done, she rubs her hand together and picks at her nails. Alternatively, if she happens to be standing instead of sitting down, she shifts her weight from foot to foot.
All of the above- for all of them. I never really assign characters specific ticks- I’ll just add what ever I remember exists at that moment in time. And it’s always twiddling fingers. Every single character I write twiddles their fingers or pinches their thumbs or messes around with their hands as soon as they get the slightest bit nervy. If a character has long hair, they probably play with it every so often, but that’s less of a nervous tick and more of a ‘I need an excuse to tell the reader that this character is a brunette’ thing.