TRADING CLOTHES & RINGING PAVLOV’s BELL by shadowkat67 is possibly the most awesome Spike meta ever. You must read it. It delves into the chip, the soul, and the trigger in a very eye-opening and convincing way. I always appreciated the way the writers on Buffy gave a nod to such things as A Clockwork Orange and The Manchurian Candidate, but this meta points out just how complete and organic those references were, among many other things. I can't even begin to understand how I missed all the amazing (and in retrospect, obvious) stuff about the trigger and Spike/William's entire existence, so you'll just have to see for yourselves. You won't regret it.