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Weight of objects is definitely something actors don't seem to be trained in anymore. Everything looks light as a feather.

A less obvious thing is tossing a drink in somebody's face wrong. Most actors toss the drink upward from the waist, which probably sends it right up their fellow actor's nose. Sometimes you can even see a reaction when this happens. Not fun. There's a definite technique that's sort of a backhand flick of the wrist, that sends the liquid straight into their face instead of up their sinuses. It also puts your hand, the glass and the face a lot closer together, so they all fit in a much tighter shot. Not that I would know.