In the D&D Cyclopedia, the guide line is, if they roleplayed to the GM's satisfaction that session, give them 1/20th of a level.
In AD&D2, and in 3E, overcoming a challenge by any means, be it RPing, combat, or puzzle-solving your way out of it, gets the XP value of the opponent.
In Holmes Basic, the following is found on page 11:
Monsters killed or overcome by magic or wits are
worth experience points to be divided among the entire
Noting the use of "wits" - that can be extended to include Roleplaying an interaction with them that results in their ceasing to be a threat.
In AD&D 1E, the following is noted on page PH 106:
Finally, clerics' major aims are to use their spell abilities to aid during ony
given encounter, fighters aim to engage in combat, magic-users aim to
cast spells, thieves aim to make gain by stealth, and monks aim to use their
unusual talents to come to successful ends. If characters gain treasure by
pursuit of their major aims, then they are generally entitled to a full share
of earned experience points awarded by the DM.
AD&D 1E DMG (Dec 1979 revision), on page 86, gives an extensive bit on how roleplaying is to be tracked, based upon class and alignment. It doesn't affect XP earned, but the effects of having earned them - it's the determinant of the number of weeks training required to advance (generating a number between 1 and 4), and those have a set cost in GP (Level x 1500gp). No training, no advancement, and you cap 1 point shy of the level after. So, not an XP award, but clearly an incentivization of role-play.