You can counter spells even when you have not readied an action.
Prerequisites
Improved Counterspell, Combat Reflexes, able to cast 3rd-level spells.
Benefit
Once per turn, you may attempt to counter a spell as an immediate action. You do not need to have readied an action or previously selected a target to use this ability.
You must identify the spell using Arcane Lore or Theology and have the relevant spell (or dispel magic) available as normal when countering a spell. This feat only changes the action necessary to attempt a counterspell.