Animosity is represented by an Animosity test. This test is taken in the Start of Turn sub-phase during each of your turns, by every unit that is subject to Animosity, with the following restrictions:
Units only test if they contain at least 5 models.
Units engaged in close combat don't test.
Units garrisoning a building don't test.
Units fleeing or currently off the battlefield don't test.
Roll a D6 for each eligible unit. If the dice roll is a 2 or more, the unit behaves normally this turn. If the roll is a 1, however, the unit has failed its Animosity test and must roll on the Animosity Table.
Characters and Animosity
Characters are not normally subject to Animosity, as they're too busy commanding the army, casting spells and such like. However, if a character joins a unit that is subject to Animosity, then he is bound to the result of that unit's Animosity test. The only exception is Black Orc characters, who get their own special rule for just such an occasion - Quell Animosity.
Charging and Animosity
Certain results on the Animosity table require units to declare a charge. Note that charges are declared and carried out in the Charge sub-phase as normal.
Mounts and Animosity
Mounts (whether a cavalry mount, monstrous beast, or a ridden monster) are subject to the same Animosity result as their rider.