From page 16 of the RCS rulebook:
"Reactive Fire temporarily interrupts the action of an enemy unit to allow one of your units
to open fire. In order to perform this action, you must first select one of your unactivated
units and roll one die. If you obtain a , your unit does not get to fire at the enemy. In
fact, if you fail your roll, your unit is considered activated for this game
From page 21:
"Using this skill does not require an action, but the hero must activate in order
to use the skill."
I really wish the forum search engine was better because I know I've seen this argued before. "Heroic Attack" only requires the hero/squad be activated. Whether your die roll is successful or not, the unit is activated. The die roll only determines if you may perform the attack action. Any skill that does not require an action could be used. Right now "Heroic Attack" is the only skill that doesn't require an action except the ones that modify your attack (beserk, spotter, etc.) IIRC.
On a related note, FFG should find a place to post all the rule questions they answer so others may view them. Or have I overlooked it somehow?