I know this is a common complain, but they have tested with special cups that have weights and special compartments to have water. Audiences can’t tell the difference. People complain that the cup is empty when it’s filled with water, or are fooled by empty cups, so much as to not matter. Unless you can see the liquid in the shot it doesn’t matter. The things like sipping or trying to move fast, are what gives away the state of the cup. Not the acting. And for those scenes there are other concerns like spilling, that are more trouble than they’re worth.