Morrowind:Disposition

Disposition is a value that determines how much any given NPC likes you. Every NPC has a default Disposition, usually set to something relatively neutral. This Disposition is then modified based on your Personality and faction rank, if in the same faction as the NPC or in whichever other faction theirs likes the least. Disposition affects vendor prices, NPC aggressiveness, how likely Speechcraft actions are to succeed, and how willing people will be to give you information. Additionally, if it gets low enough, some characters may refuse to provide you services, or in some cases refuse to speak to you at all.

Raising Disposition
There are several ways to raise a character's Disposition:
Same race as an NPC — While not a way to raise Disposition, if you are the same race as an NPC, they will have a 5 point increase in Disposition towards you.
Sheathe your weapon — NPCs don't like talking to somebody with a weapon drawn and ready, and who could blame them? Putting away your weapon (or fists, or readied spell) results in an instant 5 point increase in Disposition.
Persuasion — Characters skilled at Speechcraft can Admire and Intimidate people into being more friendly towards them. Admire is mainly based on Personality, and Intimidate works better the greater your level in relation to the subject's. Characters with low Speechcraft should be careful, however, as Disposition is more likely to go down if done poorly, and persuasion becomes less likely to work as it drops further.
Bribery — If your skill with words is not sufficient, a well-placed bribe can loosen a subject's lips and get them to like you more. However, offering a bribe they consider too low may insult the target rather than flatter them. Although bribery was intended to raise Mercantile, it only raises Speechcraft.
Charm — The Charm spell will temporarily raise an NPC's Disposition. Time is frozen while you are in conversation with somebody, so you only need a very short-duration spell to be effective. 1-2 seconds will suffice if you talk to them immediately after casting.
Personality — Using a Fortify Personality effect on yourself can also increase NPCs' Disposition. One easy way to do this is to drink a bottle of Telvanni Bug Musk. There is a glitch associated with this method, however: Any NPC who sees you while your Personality is artificially fortified (regardless of whether you talk to them) will have their Disposition lowered after the effect wears off. This may cause their Disposition to actually end up lower than it was before you used the fortify effect. In general, you should avoid contact with people who already like you while under the effect of such a spell, particularly if they are quest givers for a faction you're in—or better yet, avoid the effect completely. However, this bug is fixed by the Morrowind Code Patch.
Trading — If the target is a merchant, buying and selling items results in a small increase in Disposition. Although each increase is small, trading low-value items one at a time can be used to get a larger increase.
Topics — In certain cases with some characters, activating a certain topic will sometimes cause them to like you more. This is most evident when you complete the Main Quest.

Lowering Disposition
There aren't too many reasons why you'd want to lower an NPC's Disposition. The only major one is if you want to kill somebody without being accused of a crime. The best way to do this is to use your Speechcraft skill to Taunt them into attacking. A successful Taunt will lower your subject's Disposition, and possibly convince them to attack you. If it gets too low and they do not attack, Taunts will be less effective. Use one of the above techniques to raise their Disposition again, then Taunt again once it's high enough; every social action is more effective when the Disposition is near the middle (50), try not going below 20 and don't bother going higher than 75-80 if you only want to Taunt. One very sneaky method is to alternate between Bribes and Taunts. After you finally get them to attack, you can safely kill them in self-defense, and then take your money back off their corpse. See also: Frenzy, which has no effect on Disposition, but is another good way to get somebody to attack you.

Very Low Disposition
As mentioned above, if an NPC's Disposition goes below a certain point, they will refuse to offer services and talk about certain topics. Some NPCs may even completely refuse to converse with you, and you will be unable to use any form of persuasion or bribery to increase their Disposition. A Charm spell or higher Personality might help, or, if they belong to a joinable faction, another possibility is coming back to them after you've increased your faction reputation by completing faction quests.

One unorthodox trick that can get somebody to talk to you is to unequip all of your items. Remove every piece of armor, clothing, and jewelry you have on. Don't forget your weapon. Talking to NPCs like this will cause them to react in shock and insist that you cover yourself up. No matter, at least they're talking to you, and you can now use persuasion or bribery to raise their Disposition up so that they like you enough to tell you things. Note that this will not work for Argonians or Khajiit, nor on certain more important NPCs, notably those with the "NoLore" variable set in their scripts.

Creatures
Creatures do not have Disposition, so the few creatures who will actually talk to you will not care about your Personality or Reputation. They always treat you the same. This is particularly useful to know regarding the two creature merchants in the game, Creeper and the Mudcrab merchant. Both of them will buy and sell items at base value because their prices are not modified by Disposition like NPC merchants' are. Note that this includes such characters as Dagoth Ur, Vivec, and Yagrum Bagarn who are technically creatures, even if they seem more intelligent than your average Guar.