Stepping back for a second, it's important to remember exactly what animosity is trying to simulate. We shouldn't be making mechanics for their own sake, there has to be a reason to include a special rule. The only justification for a special rule is that the thing we're trying to represent can't fit within the existing mechanics.

The classic way of showing undisciplined troops is through low morale or bad leaders. The problem with that is orcs aren't cowards, they're just uncontrolled brawlers.

So what we really are looking at is a way of showing that orcs are really rebellious against their own leaders. At the same time, once fighting gets started, they're no worse than anyone else at staying with it. The core problem is keeping them occupied until they can engage the enemy.

From a mechanics point of view, the less dice rolling and charts, the better. A simple "if-then" kind of rule is the way to go with minimum randomness.

My current thought is therefore that animosity takes place on a 1 or a 2 on a d6. Roll at the start of movement when charges are declared. If a unit is affected, go down the checklist:

If it is within charge range of any unit (friend or foe) it immediately charges. Preference can be shown to the enemy, but if more than one enemy is there, the target is randomly selected (the boyz go where they wanna go).

If no unit is within charge range, they sit still and fight each other, doing nothing that turn. They can still react to a charge, but otherwise remain immobile.

HOW TO AVOID ANIMOSITY: I've come up with two ways you can short-cut animosity.

The first is the presence of the Army Commander. Any unit personally led by him in immune so long as he's alive. His commanding (and dangerous) presence keeps the troops in line.

The second way is to crack some skulls together. At the start of the turn, the orc player can opt not to roll for animosity by voluntarily "killing" (removing) a model from an otherwise eligible unit. Basically, the unit commander kills a few punks to keep the rest in line. It's bloody, but it works. There's no penalty for the unit (other than the loss of a model) and it can charge, move, shoot etc. normally.

Only infantry can do this. Everyone else (cavalry, monsters) have to roll.

I think this works because it encourages players to get stuck in right away but also gives the more ruthless among them a way to keep their troops in line. I also like the fact that animosity is less of a fluke. A 1 in 6 chance is low enough that it may not come up but when it does (and does so repeatedly) it feels more like bad luck than something the player should have anticipated.

I'm sure some folks will object that animosity can be eliminated through judicious use of killing, but that's actually true to the source material. When orcs are encountered on LOTR, they are either in abject terror of their masters (the Nazgul, the Balrog) or are kept in line with some well-timed beheadings.

I think that option is therefore colorful but also easy to understand and implement. Plus, in big battles players can keep track of friendly vs hostile losses.