How often to units deviate from scripts?

[13lackGu4rd]

as for your ogiginal question, how often, generally if your scripting is good than never. however scripting can be faulty for a variaty of reasons: no targets in range, forgot to bring gems to commanders(for spells that require gems that is), already having the effect(such as your charge body example), the enemy being immune to your spell(like scripting cold attacks against an all undead army), etc.

[Illuminated One]

The having the effect can also be in place when you only technically have the effect with not enough to benefit.
For example if you script an eagle king (100% shock resistance) to ironskin (-75%) they have 25% shock resistance. Meaning still toast in one hit. But they will ignore any order to cast lightning resistance. Don't know if that's limited to lightning or to units starting with 100%.