> If you look closely, you'll also see the coffee/tea cups actors sip from are usually empty. Can't afford the risk of accidentally spilling liquid on the costume and delaying the shoot.

If I were a prop-master (is that what it is called?) I've always thought that I'd just have a bag of plaster of paris handy. Then 30 minutes before going on set just dump some in the prop-cup with some water.

Sets quickly, density is about the same, physics of the cup should look convincing. Probably best for disposable cups though.

Scott Reeder (who is a prop master and makes short videos on about his craft on various platforms) has weighed down disposable cups a couple different ways. One was dropping a mini water bottle into the cup, and another was dyed food grade silicone (for when the shot required a cup with the lid off).