Niche D20Most spells require 4 AP to cast, but there are exceptions. Certain spells or abilities may increase the time required to cast a spell. Some effects may increase the AP required to cast a spell. Spells reduced below 1 AP are instead cast as a reaction as a minimum.
Spells cast as a reaction do not provoke attacks of opportunity, though still require concentration and as such are susceptible to any other factor that could potentially interfere with casting a spell.