Incidentally, there is a buff icon
that appears before he casts it. I've accidentally hit my interrupt right when I saw the icon, but before the cast bar started, and it actually interrupted it.
i was wondering how my friend on his tank interrupted it so fast. He told me he could tell by the animation, but I could never pick up the clues before the castbar started. Doing HM TfB with him tonight, thinking someone will be getting less heals than normal.