A player in my group wants to take the Lightning Reflexes virtue, and choose "cast my signature spell" as his default action. The virtue says that
Do you think that "casting a spell" is a valid "type of action"? Does it matter if the spell is mastered for fast-casting? If he can cast a spell this way, does he get the +9 bonus to Initiative?
Yes I think he could choose Lightning Bolt as response, if he fast casts. And yes the Init bonus should apply.
It's very dangerous but just as valid as "I stab it".
Also I don't think the player needs to pick just one response all the time; just that it can't be a protracted action.
Thanks.
He doesn't need to stick to one response, but he needs to decide on the response before he knows what happened so it's simplest to decide on a default action in advance, and change it as desired in-game.
Spellcasting in combat has an initiative total, just like any other action. It's equivalent to the character's QUIK score. Thus, Lightning Reflexes could apply.
I'd allow him to use it for fast cast defence also.
I'd argue that a momentary "dome of stone around myself" is probably a more useful spell to act as a general counter, in that it's less likely to shank the cat by accident.