I played thrash for about a decade. I used a pick to cut through and tons of volume to compete with the guitars. I tried being different than just using fast root notes, but thrash is pretty much based around speed. Cliff Burton is a cool player, but his best parts to me were the slower melodic ones. He gets buried by the chunk of old Metallica. I loved early Megadeth because the bass was very complimentary and had different lines than just repeating the guitar parts. I love early Maiden too, which is the precursor to thrash really, and Steve Harris plays all sorts of great stuff that cuts through. I suppose my advice would really be to explore your neck more than "holding it down" for thrash. You will be playing those thick parts enough in that style. You can be really effective by playing an arpeggiated lick underneath a chord, or a walking type run instead of all the notes. Many thrash bassists would "cheat" as I called it, and play one note to a flurry of guitar notes. These were mostly finger players, but it can be really effective too. I would refer to maybe the band Death's early stuff for a good example. As for theory, learning the guitar chords you are playing over really helps. You can play a 3rd, 4th, 5th, or octave note to complement the chord and this can really increase the tension, or color the sound more. I would also say just experiment and don't limit yourself to what I say or anyone else. Pay attention to the drums and guitars and lock in with both to give you more ideas. Play what sounds good and feels good to YOU and don't let all the rules limit you. Get very loud gear and crush those guitars!
