A further clarification on the rules described in the Magic & Damage section:
‘You may launch a counterspell, providing active opposition to any spell being cast that you are aware of regardless of the cost demanded by that specific magical effect.’
If a stunt gives you permission to employ a counterspell you are simply providing active opposition to an action that creates a magical effect where normally no active opposition would feature. As stated, the mage attempting to cast the spell will be forced to roll dice, actively opposed by a player or Dungeon Master even if he wasn’t using the Create an Advantage action. So, for example, if a mage is attempting a summoning spell that costs a fate point he will be forced to roll dice as well if opposed by a mage with the ability to counterspell.
The DM should provide exceptions for certain magical effects where sensible, for example when magical items like scrolls and wands are in use. In the case of magic missiles and the like the counterspell should allow the target mage to make a defend action with his spellcasting profession rather than Acrobat.
This of course isn’t the only case where a mage may be faced with active opposition. For example a pack of frenzied Gnolls who have chosen to gang up on a priest who is a clearly a magic user might create the aspect Gnolls Breathing Down My Neck!, which the priest would have to overcome before casting any spell. The DM may rule in this case that removing the aspect will be actively opposed by the Gnolls.