If a multiclass character's classes are not nearly the same level, then he suffers a multiclass penalty of -20% XP for each class that is not within 1 level of his most experienced class. These penalties apply from the moment the character adds a class or raises a class's level too high. For instance, a 4th-level wizard/3rd-level rogue gets no penalty, but if that character raises his wizard level to 5th, then he'd receive the -20% penalty from that point on until his levels were nearly even again. The penalty is -40% if three classes are uneven -- for instance, a 5th-level wizard/3rd-level rogue/1st-level fighter -- and -60% if four classes are uneven (a 7th-level warlock/5th-level wizard/3rd-level rogue/1st-level fighter).
Races and multiclass XP
A racially favored class does not count against the character for purposes of the -20% XP penalty. For instance, an 11th-level gnome character (a 9th-level rogue/2nd-level bard) suffers no XP penalty, because he has only one nonfavored class (bard is favored for gnomes). Suppose he then achieves 12th level and adds a level of fighter to his classes, becoming a 9th level rogue/2nd-level bard/1st-level fighter. He suffers a -20% XP penalty on future XP he earns, because his fighter level is so much lower than his rogue level.
A dwarf 7th-level fighter/2nd-level cleric suffers no penalty, nor does he when he adds 1st-level rogue to his classes since his cleric and rogue classes are only 1 level apart, and fighter is a favored class for dwarves.
Human is not always the best choice to avoid an XP penalty: For example, a human 2nd-level fighter/8th-level wizard/8th-level rogue would suffer a -20% XP-penalty, a dwarf with the same class combination would not. Further, since NWN2 now allows up to 4 classes to be chosen, Humans and half-elves can now suffer a -40% penalty with some class combinations, and other races can suffer a -60% penalty (e.g. a dwarven 7th-level wizard/5th-level cleric/3rd-level rogue/1st-level warlock would be under a 60% cumulative XP penalty).
Prestige classes and multiclass XP
Prestige classes are exempt from the multiclass XP rule and do not count against the character. For example a 20th-level character (10th-level rogue/3rd-level shadowdancer/7th-level assassin) will not suffer any XP penalties no matter what race the character has chosen.