MechaTarrasque
Hero
I tend to use alignment for NPC's. Players can use it aspirationally (my PC desires to be as LG as he can be), and I will try to set up circumstances to test/move forward with that goal. Other than that, the world reacts to the PC's as they act (or as NPC's paint their actions). I have no problem having a bunch of angels attack the murderhoboes calling them "vile fiends in human form" if that is how they act (and likewise have demons tell them "I have heard great things about you, keep up the good work.") That is the limit of my enforcement.
As for definition, I tend to go with law="respectability is more important than renown" and chaos="renown is more important than respectability" for simplicity's sake. Good and evil is a little more nebulous, although if pressed it tends to go something along the lines of evil="enjoys hurting others", where killing quickly doesn't count as enjoying hurting others, N="would enjoy it if exceptionally provoked (if someone killed your family)", and good="wouldn't enjoy hurting others even if provoked." [Part of the reason fiends do bad things is if you start to enjoy hurting them, the slippery slope begins.....] I also tend to use LN with good tendencies, NE with lawful tendencies, NE with chaotic tendencies, etc., based on the notion that everyone has a primary motivation that isn't likely to change, and a secondary one that could more easily change.
As for definition, I tend to go with law="respectability is more important than renown" and chaos="renown is more important than respectability" for simplicity's sake. Good and evil is a little more nebulous, although if pressed it tends to go something along the lines of evil="enjoys hurting others", where killing quickly doesn't count as enjoying hurting others, N="would enjoy it if exceptionally provoked (if someone killed your family)", and good="wouldn't enjoy hurting others even if provoked." [Part of the reason fiends do bad things is if you start to enjoy hurting them, the slippery slope begins.....] I also tend to use LN with good tendencies, NE with lawful tendencies, NE with chaotic tendencies, etc., based on the notion that everyone has a primary motivation that isn't likely to change, and a secondary one that could more easily change.