Your probably going to get alot of opinions on this one but I'm thinking that your definition of FAST could probably be put side-by-side with spontaneous, right ?
No thought, no RE-action, just action. If the particular technique your in doesn't work or your attacker is resisting, simply move into another one.
I think that's very possible but I also think that people have "familiar" or maybe "favorite" techniques that they will use -or attempt to use- when a situation arises. That, I don't think can be helped without several years or maybe even decades of practice.
It's simple human nature to do -or try to do- what we are most comfortable with in any given situation.
From a more scientific standpoint, it's a conditioned reflex that probably comes from one of the lower brains.
That's one of the things that I love about Aikido. There's no absolute way of doing any one technique, that is, nothing is etched in stone as far as form is concerned and that gives most (if not ALL) of the rest of the techniques the ability to implemented almost anywhere.