I just started playing Baldur's Gate II again, having gotten stopped last time well before I finished it. When I created a new character and had to choose her alignment, the game gave me a dire warning about the "consequences" of straying from your alignment during the course of the game. I was wondering what exactly these consequences are? (I wanna tell Jaheira to beat it, but I'm Lawful Good and I'm afraid to because of the aforementioned dire warning. ;) )

Nothing, aside from some minor dialog with various NPC, and some quest that only evil PC can do (which every evil character worth his salt wouldn't do anyways).

You do not get a penalty for kicking Jaheira from your party as a paladin, just tell it the way it is. You have to prioritize, after all, there is a limit to how many people can tag along in your party, right? Lawful good characters follow the law and treat other people kindly, but it does not necessarily have to be an evil action not wanting to have certain people in your party.

No, evil actions are hacking innocent civilians down in the streets and the like. Now THAT is unacceptable behaviour, especially as a paladin.

I honestly don´t think that any conversation throughout the game can affect your alignment to such an extent, at least not to my knowledge. But then again, I don´t know what happens, if a paladin tells a beggar to piss off. Certain trials and tests later on CAN change your alignment, however. Now that will put you to the test. Does your paladin have the power of his conviction? Or is he a selfish bastard, who just has´nt realised it yet?

Besides, characters like Edwin and Korgan are both Evil. If you ever got any of them in your party.... hmmm.... would it be considered an evil act to tell them to bugger off? I don´t think so. You just go ahead and tell her to shooo.

However, I would´nt, she´s a great addition to any party, good and evil, being both a spellcaster and a fighter.

The only classes that need worry about straying from alignment are Paladins and Rangers, which both become "Fallen" if you perform evil acts to the point that one's reputation drops.

Also, if you stray from party alignment, some NPCs will leave in disgust.

^^What Rednik said.

In addition, if you take a Cleric stronghold but do not act according to the tenets of your faith then it is not quite as beneficial as it could be. Though... still not really a penalty.

Whether or not there are tangible consequences, it's still worthwhile trying to stick to your alignment while you play.

Yes, it's a matter of playing in the spirit of RPG.
Note that some NPCs stray from their own alignment.

@Greystar Nothing, aside from some minor dialog with various NPC, and some quest that only evil PC can do (which every evil character worth his salt wouldn't do anyways).Having never played evil, could you spoil me on this one?

An Evil PC (or it might be based on Reputation) gets the opportunity to poison the Druid Grove near Trademeet.

Also, an Evil person might kill Adalon and use the skin found in the Tanner's Shop with her blood to make some human skin armour.

EDIT: Oh, there's a kidnapping involving some people from the Bridge District that Evil people might want to get involved with, too.

As an evil character you can make a deal with the devil. Terrific stuff. Sadly (or not, I guess it depends on how you think a game should be made), all quests in the game are skewed toward good-aligned solutions. Poisoning the Druid Grove is a pretty decent evil solution, but the reward is crap compared to finding a peaceful solution.

Basically, there is no balanced in-game reward for playing your alignment properly. Good thing or a bad thing, you make the call.