Curious as to how someone is able to interrupt a caster if the spell takes place on the caster's initiative, unless they are readying an action. If a spell is declared on initiative and then goes off on the same count a round later, this would seem to follow the spirit of the rule that a spell takes 1 round to cast. However by RAW this is not the case. Anyone have any ideas?
Well, we don't know a lot, but we know some. Fast-cast spells must be able to go off sooner since you can get them to affect things before your initiative. Also, since you can do a fast-cast spell plus a normal spell in one round, each must take less than the full round. You can do many fast-cast spells in a round if you're skilled enough, so presumably they take noticeably less time than a normal spell, plus the name implies as much. Multiple-cast spells do not take less time; in terms of computer programming, it can be stated that these spells are 100% parallelizable and optimally so. Since multiple-cast spells do not take less time, these rules don't imply how much time that is. Beyond the minimal amount I've said, I don't think I can say anything more definitive.
I've seen HRs that enforce all spells must be fast cast within the round... IMO, this isn't necessary if you treat fast casting as reactionary and (primarily) defensive, and don't get into situations where you fast cast a BoAF at an opponent, after having lost initiative to beat your initiative and take him out before he takes you out. Fast casting Wizard's Leap or sidestep, or something that is defensive, rather than offensive gives fast cast rules a reason to exist.