Interrrupting is all a matter of reflexes and intuition. You need to be able to quickly recognize the casting animations and click your ability on them, or innately know the characters and be able to accurately guess when they may use their abilities. Regulus's big giveaway for mines, for me, isn't the hand going back,but the fact that he's stopped the autoattack animation. As soon as I see that, you know he's going for the mines in his pocket. Oak is the same. He stops attacking, then lofts the axe. The rest of the characters, like I said, comes down to intuition and a little blind luck. Watch their combat and abilities cycles, try to guss tics, and sometimes you can even nick an Erebus' Bite or similar. I've gotten lucky and Pounce-interrupted another Sedna's Pounce before.
Practice practice practice and use your eagle eyes. TB's deep freeze IMO is the best interrupt in the game (ranged, instant. Even Pounce and graps suffer from being melee, or in pounces cast a half second (?) cast time, until the cat actually hits them), so get good at it.
