A less specific version of my
Skull Bash macro...
electrochem and I were talking about a focus macro that would let you interrupt your focus target, if one existed. If no focus target exists then you'd just interrupt current target. It would also clear your focus target if it didn't exist, or was dead.
#showtooltip
/stopcasting
/clearfocus [@focus,noexists]
/clearfocus [@focus,dead]
/cast [@focus,harm]Counterspell;[@target]Counterspell
Replace Counterspell with your class interrupt (Rebuke, Pummel, what have you)