Is this for PvE or PvP purposes? In PvE, most encounters only require you to really be on the ball on one target when it comes to interrupts (Arcanotron, Maloriak). I use focus frames for those in conjunction with focus interrupt macros, and most cast bar addons like Quartz support focus cast bars. For easy focussing your mouseover target, I use this (keybound) macro:
Code:
/focus [target=mouseover]
in conjuction with
Code:
#showtooltip
/stopcasting
/cast [target=focus, exists] Interrupt spell name
Hope that helped