Note: This is just cool picture one of my movies, not necessarily having to do with bad acting or directing.
Rule #2. Bad lines and shallow character is due to bad script writing, and once again to bad directing.
Rule #3. Directors can make good acting look mediocre by not backing it up with the shot "other". Inappropriate lighting, poor sound, bad location, poor choices in shots and angels.
Rule #4. Editors can make good acting look terrible by choosing the wrong takes.
Extra Rule For Actors #1. Actors are not paid to act, but to hurry up, and wait around to act.
Naturally, some actors are better than others. But in film more than any other medium (like stage for eg.) even the worst actors can be made to look good, and the best actors made to look terrible.