I give them the choice to have their alignment switch- which may affect the PC mechanically if it's a Pally, Monk, Bard, Cleric or the like- or act within the character's alignment.
I've also been given the choice...and gone different ways, dependin upon the PC I was playing.
Thanks for saving me part of a response. haha. This is how I would do it too...and even the option is only brought up after a string of observed behavior taking place over time (real and game).
As a DM,
for me, it's not like, "You just cursed in front of that old woman. CHAOTIC EVIL!" ...well, maybe a paladin. hehe.
I kid! I kid.
To reiterate someone's point, we're all entitled to a "bad day" or a "being in a mood" and that goes for characters as well as players. We don't always react the same every time to given the same or similar circumstances or stimuli.
More over, a player going through hard times may be likely to have their character take out their frustrations as a kind of cathartic therapy.
To use the example from my previous post, the "Chaotic Good" ranger was being thoroughly and unnecessarily cruel and nasty over multiple sessions...with monsters, NPCs AND the party members sometimes. Finally, after "letting it slide"/giving him the benefit of the doubt and several warnings, I had to say, "enough."
The player was going through some personal stuff, but that's not the group's fault or the party's fault...and his actions and attitude were beginning to interfere with other people's enjoyment. We made allowances, all being friends out of game and all. But when it was going on over a month, and several conversation with group members later, the detracting from the mood and fun of the whole group was unacceptable.
Pull it together and play or excuse yourself from play for a time (totally understandable and acceptable sometimes).
Having to change a character's alignment is, by no means, a common occurrence in my DMing experience.
--SD