One thing I noticed over and over, from cheap sitcoms to expensive blockbusters - when actors sip, they have liquid, but clearly the movement of glass to mouth is 'dishonest', as in too fast or too low for any liquid to actually make it into mouth. No swallowing movement of throat neither.

I guess its subconscious - they know they are not going to actually drink it, they focus their mind on other aspects of acting, so this part leaves them not faking it well.

If you see it once, you can't stop noticing it elsewhere afterwards, beware.