One of my favourite little details in a film is Ash (Ian Holm) running on the spot in Alien, when nobody else can see him. At the time, it looks like a quirky way of gearing himself up to do work, and fits in with his slightly neurotic, prissy personality. On subsequent viewings, it's more likely that he's got some minor internal problem, and is checking his systems. The excellent computer game Alien: Isolation includes primitive robots that, when left to their own devices, occasionally do the same thing.