Do interrupts and punishers mix?

Say you cast backfire on a caster, then interrupt every spell he casts. Does backfire still hit him each attempt, or is it wasted? What about similar skills like Air Attunement - if you interrupt his spell, does he still get 30% of the energy back from Attunement?

wow thats an interesting question I'd like to know myself....

Because when I used backfire I always just used dots to try to get him as low as possible so if he does accidently use a spell BAM dead. So I never interrupted or watched if backfire worked right when he uses the spell or after he casts it. I think I do remember seeing it backfire on someone after they casted a long spell. So probably if you interrupt someone it wouldn't trigger backfire?

Backfire only deals damage when the target has finished casting a spell.
Hence if you interrupt them, they won't take damage from backfire.

i can confirm the above.

^ as can I.