For a Dm to tell a player, "no, you can't do that with your character" or "Your character wouldn't do that!" in regards to your character's personality?
No way. DMs should back off. Its the players right to play their character. I would be very miffed with any DM who said "your character wouldn't do that" for any reason. (Well, maybe if I was mind-controlled, but then thats not me running my character, its my character being mind-controlled.) As a DM, I would never say: "your character wouldn't do that". Hmmm.... what might I say?
Example: Paladin lies to his lord to escape punishment for a crime. "OK. Your lord accepts your story b/c he knows your honor is beyond question. Later that night, you feel a twinge of guilt. The next day, the sky seems cloudy and overcast. You feel a weight of guilt on your heart. Your mount looks at you heavily and turns his head. You find that your magical abilities no longer function."
Example: Established Chaotic Good lord suddenly issues a decree that the religion of Saint Cuthbert is outlawed. "Your decree goes out across the land. A few days later, the high priest of the church of Trithereon (CG deity) seeks an audience. He implores you to rescind the decree. He says that the people should be free to choose their own path, even if they choose to follow a strict deity like Cuthbert. He says it is wrong to impose your religious views upon the people."
Example: Evil Assassin character refuses to kill people just for money, and only accepts missions against evil characters. "You awake to find a message from your guild scrawled in blood on a sheet of parchment and nailed to your bed. The message says: You WILL accept the next job or find yourself permanently retired."
Example: Monk violates the precepts of his monastery by drinking wine and carousing with women at the local bar. "Your master requests a meeting. 'You have fallen from the path of enlightenment. I fear the lure of temptation has proven to be too much for you. How can you find perfection if you indulge in your baser pleasures? You shall be given one final chance to purify your heart and continue your education. There is a task you must complete...'"
Example: Druid burns a small forest to clear land so a new city can be built. "The next day, your animal companions are not around. Your owl says farewell before it leaves - you sense great sadness from it, as though you had betrayed its trust. You sense a disturbance in the natural order in this place, and know that it was your actions that brought this about."
I would never tell the player that his character would or wouldn't do something, but if their character violates their beliefs or philosophy, there will be logical consequences. Maybe this will lead to a redefinition of the character - monk moves to barbarian, or paladin becomes a blackguard - or maybe it will lead to redemption as the player further defines his character and comes to understand what truly motivates him. In any case, its the players job to play his character however he sees fit, and its the GMs job to respond to that and run the world consistently.
He introduces himself to everyone as something different. "Hi, I'm Ted. Ted Nugent." "Axelrod, Axelrod Rimthruster, glad to meet you." "Helena, Helena Handbasket." Angcuru looks at me and says "no, you can't do that."
Now on the other hand, that's just being silly. Everyone in the game has a responsibility to contribute to the game to make it fun and consistent. If the GM is trying to run a "serious" game, and you're trying to be silly, then you need to come to an agreement about how to play the game. If you want to use names that are consistent with the feel of the game ("Hi, I'm Drizzt Darkwing", or "Hi, I'm Torym Angwellyn") then that's cool, b/c you're not butting heads with the GMs expectations for verisimilitude.
On the other hand, the GM shouldn't be saying "No, you can't pretend to be someone else or lie about your name." My response would be: "Why not?" The GM also wouldn't be handling it well to say "No, you can't do that" - it would be better to say: "Please don't use silly names. Try to keep it grounded in the fantasy reality. For example, if your character wants to conceal his origins, try to use names that your character thinks would be good disguises. Don't say you're Elminster, since no one would believe that."
What's your opinion, is it fair for a DM to do this? I'm gonna play the character as a dead-serious ranger the rest of the time; I just want to lighten up the mood a bit sometimes, and it does seem like it'd be appropriate to the character.
Check with your DM to see how where he stands on "lightening the mood" - humor's touchy, and one person's funny is another's annoying. You might think its hilarious to say your druid character claims to be "George Bush", but the GM may have strong feelings about keeping real-world references out of the game. Don't piss off the GM for a cheap joke. But do insist on the freedom to play your character within the tone and tenor of the game that the GM wants to run.
Ozmar the Opinionated