You can change the attitudes of others (nonplayer characters) with a successful Diplomacy check; see the Influencing NPC Attitudes sidebar, below, for basic DCs. In negotiations, participants roll opposed Diplomacy checks, and the winner gains the advantage. Opposed checks also resolve situations when two advocates or diplomats plead opposite cases in a hearing before a third party.
Action: Changing others' attitudes with Diplomacy generally takes at least 1 full minute (10 consecutive full-round actions). In some situations, this time requirement may greatly increase. A rushed Diplomacy check can be made as a full-round action, but you take a -10 penalty on the check.
Try Again: Optional, but not recommended because retries usually do not work. Even if the initial Diplomacy check succeeds, the other character can be persuaded only so far, and a retry may do more harm than good. If the initial check fails, the other character has probably become more firmly committed to his position, and a retry is futile.
Special: A half-elf has a +2 racial bonus on Diplomacy checks.
If you have the Negotiator feat, you get a +2 bonus on Diplomacy checks.
Synergy: If you have 5 or more ranks in Bluff, Knowledge (nobility and royalty), or Sense Motive, you get a +2 bonus on Diplomacy checks.
The character can turn a person into a fanatic follower. Refer to the accompanying table.
Influencing NPC Attitudes
Use the table below to determine the effectiveness of Diplomacy checks (or Charisma checks) made to influence the attitude of a nonplayer character, or wild empathy checks made to influence the attitude of an animal or magical beast.
Fanatic: In addition to the obvious effects, any NPC whose attitude is fanatic gains a +2 morale bonus to Strength and Constitution scores, a +1 morale bonus on Will saves, and a -1 penalty to AC whenever fighting for the character or his or her cause. This attitude will remain for one day plus one day per point of the character's Charisma bonus, at which point the NPC's attitude will revert to its original attitude (or indifferent, if no attitude is specified).
Treat the fanatic attitude as a mind-affecting enchantment effect for purposes of immunity, save bonuses, or being detected by the Sense Motive skill. Since it is nonmagical, it can't be dispelled; however, any effect that suppresses or counters mind-affecting effects will affect it normally. A fanatic NPC's attitude can't be further adjusted by the use of skills.
Source: SRD 3.5 Combined Skills