What exactly interrupts spells?

[saber_alter]

I actually think high damage isn't a factor. The reason for this is because certain high-damage skills do NOT interrupt spells, such as a primal's limit break. Hellfire will never interrupt your spell, even if it does 90% of your health. Mistral Shriek, on the other hand, will always interrupt your cast, even if it does very little damage.

I suppose it depends on what the spell is coming from. as you said, certain attacks from primals have either a very low-to no chance or a have an almost 100% chance, but i think on other occasions damage is a factor.

take turn 1 for instance.
at 0-2 stacks caduceus rarely interrupts when i cast stoneskin on myself, whereas 3-4 he interrupts them half the time, and higher even more-so.