Yeah, for the record, I would assume from what I know of how
@iserith DMs, at their table it wouldn’t be that fidgeting is a reliable indicator that any character is lying. It would be a telegraph that
this specific character fidgets as part of one of their background characteristics. Perhaps a Personality Trait or a Flaw. If you act on that, you might be able to determine what that characteristic is.
For example, I might try something like “As we’re talking with him, I pay close attention to when he fidgets to try and see if there’s any noticeable pattern behind it.” I imagine
@iserith might then either tell me I notice a pattern and what it is, tell me I don’t notice any such pattern, or ask me to make a check of some sort. At a guess, maybe passive Wisdom, since it is an action my character would be continuously engaged in throughout the course of the conversation, to which I might suggest I add my proficiency bonus for Insight or Investigation if I have either skill. And if whatever trait is behind the fidgeting were to come up again before I announced a different action,
@iserith might make a check for the NPC - perhaps Charisma (Deception) if the fidget is a tell and the character lies again, and if it fails to beat my passive Wisdom (+prof if I had a relevant one),
@iserith would inform me that I recognize it’s a sign that he’s lying.
Just a hunch.