What exactly interrupts spells?

When monsters attack you (as it happens, in particular to healers), your spells sometimes interrupt. I haven't been able to figure out a distinct pattern of what interrupts them. The game doesn't teach you either. Sometimes monsters wail on me for dozens of swings, and I'm fine. I can take spell damage, I'm fine. Other times it just interrupts. I never know if it's safe to stand and just heal, or run to heal from a distance.

So, what exactly causes it? Certain spells? How can I tell which ones? Attacks that hit for a certain amount of HP? I've been hit by strong attacks and been able to heal though.

Also, there need to be a few more cues that your spells got interrupted. When things heat up, I find myself spamming keys just because I want to make sure that my spell triggers. Certain games use audio for that. ;3

As far as I can tell it's a somewhat low chance on regular attacks, higher chance if not 100% with skills and very high chance from a crit. I don't have numbers but this is what I felt as a WHM from the getgo

Hmm, interesting. Though, 100% from skills can't well be, because in solo play you get hit by spells a lot. Or do you mean the spells with the AOE graphics that you have to run out of?

Is that documented anywhere? I've been looking on Google, but all I find is one post where someone got confused that moving interrupts their spell.

What I know so far:
1. High damage input will ALWAYS interrupt your cast, so I'd say there's a threshold there
2. Your client de-synced with the server, causing Time-Shifted Skill Rotation (tm, patent pending in SE legal department) where you have stopped moving and started casting in client but the server sees you as still moving and just stopped. The synchronization forces you to stop, the last known status of you by the server. This effect also manifests as you executing a skill (such as Windbite) and performing the animation correctly but the effect doesn't get applied and the skill is still available for use (not in cooldown)

There's also things with stun attacks interrupting you, but that's a given already

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.

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Originally Posted by Zaft

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.

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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.