Interrupting FX Users
Distracting Spellcasters and Psionic Characters: A character can ready an attack against a character or creature with the trigger “if he or she starts casting a spell or manifesting a power.” If the attack succeeds in damaging the character or otherwise distracting him or her, he or she may lose the FX ability he or she was trying to use (as determined by his or her Concentration check result).
Readying a Counterspell: A character may ready a counterspell against a spellcaster (often with the trigger “if he or she starts casting a spell”). In this case, when the spellcaster starts a spell, the character gets a chance to identify it with a Spellcraft check (DC 15 + spell level). If the character does, and if he or she can cast that same spell (are able to cast it and have it prepared), the character can cast the spell as a counterspell and automatically ruin the other spellcaster’s spell. Counterspelling works even if one spell is divine and the other arcane.