Maybe the person you interviewed was on the autistic spectrum (or had ADHD) and the finger drumming was a form of stimming? https://en.m.wikipedia.org/wiki/Sti

Maybe the person you interviewed was on the autistic spectrum (or had ADHD) and the finger drumming was a form of stimming? https://en.m.wikipedia.org/wiki/Stimming https://en.m.wikipedia.org/wiki/Stimming I have no idea since I wasn’t there when you interviewed that person but it is something that came to mind.