Creatures use the Diplomacy skill to influence others using tact, subtlety, and social grace. (Monsters rarely make Diplomacy checks.) Make a diplomacy check to change opinions, inspire good will, haggle with a merchant, demonstrate proper etiquette and decorum, or negotiate a deal in good faith.

        Action: Standard action. A Dungeon Master might allow a creature to make a Diplomacy check as a free action.
        DC: The Dungeon Master sets the DC using the Difficulty Class by Level table. The target’s attitude (friendly or unfriendly, peaceful or hostile) and other temporary modifiers (such as what the creature performing the check is seeking to accomplish) might apply to the DC. The DC might also be affected by the number of targets the creature is trying to influence at once.
        Success: The creature achieves the desired influence. This might be the first of several successes—perhaps part of a skill challenge—required to fully influence a target.

        Comfort a distraught person (moderate DC)
        Display proper etiquette at a formal event (moderate DC)
        Give a pleasing speech (easy DC)
        Give an inspiring speech (hard DC)

Published in Player's Handbook, page(s) 183, Rules Compendium, page(s) 142.