I've been an active shooter and hunter for more than 50 years. I have a pretty good library of reference materials on firearms, history, etc., and try to keep abreast of new developments in the field. I recently retired from the corporate meatgrinder, and began part-time work at the LGS.
Things I've learned:
1) Check each weapon to ensure it's unloaded, before handing to a customer (lots of them don't, and muzzle discipline is too often nonexistent).
2) Check to ensure unloaded before returning to the display case (just in case the next employee forgets....).
3) If advice is sought, offer it. If the customer is absolutely, positively determined to purchase a .500 S&W for concealed carry, just sigh and do the paperwork.
4) If a customer enters the store and heads directly to a specific firearm or genre (especially AR 15s), it's almost certain that customer knows more about that particular model / genre than I do. It's simple: Customer wants model X. Customer performs extensive research (Internet, magazines, fellow shooters, etc.) and learns about model X from A to Z. I have to maintain familiarity with all the rifles, handguns, shotguns, ammo, components, reloading gear, accessories, etc., the LGS offers. Just don't be too proud to admit I have no idea whether the 16.5" M4-config barrel from manufacturer A is as good as the 16.75" lightweight barrel from manufacturer B ...
Most customers are great to deal with. Some I worry about. I have only encountered one "bad" customer. He was furious about being delayed by NICS; got loud and abusive. The manager and I invited him to do business elsewhere, and he never took possession of the handgun he wanted to buy.
It is definitely a different world, behind the counter. I'm fortunate that I only do it to keep engaged and active. The day it stops being fun, I'll tender my resignation.