Rule 2: Never let the muzzle cover anything which you are not willing to destroy.
If there was a single gun rule that, when violated, resulted in the most damage, injury, and death, it would have to be Rule 2.
After all, the direction a gun is pointed determines where the bullet goes when the gun fires. While Rules 1 and 3 are devoted to preventing the gun from going off unintentionally, with Rule 2, it doesn't matter whether the gun is fired intentionally or unintentionally (or, as the anti-gun crowd believe, it goes off by itself). This Rule must be followed, or bad things happen.
Rule 1 is a philosophical rule. It takes a change in the shooter's thinking to make it work. You need to have a change in your attitude, so that you always treat the gun as if it were always loaded, even if it isn't. Which it is - see Rule 1.
Rule 2 is a concrete rule. It doesn't matter how you think about the Rule, what your attitude is. The gun is pointing where the gun is pointing, regardless of how you think about it.
In my experience, Rule 2 is the most violated of all the safety rules. The main reason for this is ignorance and inexperience. New shooters may be used to playing with a toy gun, or not having a gun around at all.
And then there are the thousands of very bad examples on TV and in movies.
Wow. Not only did Vincent violate Rule 2, he violated Rule 3. Bad things happened. Sorry, Marvin.
Almost as bad are the hundreds of images where an actor points the gun in the air as they move around. No one tells viewers that this isn't for good safety practice, this is so the gun is in the shot with the actor's face.
This Rule takes concentration. Nothing about Rule 2 can be left to attitude or thinking or philosophy. You must be aware where your gun is pointed at all times, or bad things can happen.
So, what works when teaching Rule 2? Zealous enforcement and repetition. When you find someone waving a gun around, call them on it. Whether you are polite about it or not depends on the situation and the violator, I suppose, but you should be firm and unequivocal in any case. In my family, we are all allowed to call "MUZZLE!" and push the gun downrange or toward the ground if it's safe to do so.
A search of most gun forums will also find many accounts of violation of this rule at gun ranges, gun shows, and gun stores. When a polite but firm reminder not to point the gun at something they aren't willing to destroy goes unheeded, the only recourse then is to leave. I know I've done it.
Rule 2 is the failsafe of the gun rules. Follow it - make sure the gun isn't pointed at anything you aren't willing to destroy - and even if someone violates the other rules, you will be safe.