ok i want to go from a pretty much metal/metalcore (metallica, as i lay dying, in flames) to playing a more like bands like (fall of troy, protest the hero) wat is the key behind the complete sound difference that i could use in my own writing
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I've tried to do this before as well, I found that the easiest way to go about it is to learn a LOT of music from the bands that you're trying to shift your sound towards. Once you start learning the music, compare the two styles, watch videos, break it down theoretically, listen to the blend of how the instruments work together and exactly what the guitar is doing to effect it. There's no EASY way to go about it if you want to do it right, it just takes lots of compare/contrast.

I find watching videos of the band playing the songs very helpful as well, subtle changes in picking style and fretting style can change the feel of the music.
It's not really the style of music you play, it's all in your fingers.

I was playing some rock/alt stuff, and some guy told me i make it sound really bluesy. I wasn't trying to; it's just the way i play, the way my fingers worked.

As for writing, listen, listen, and listen to all those bands and the music you were talking about. Don't forget about the mood and feeling of the song. If you're feeling metal, that's how you're gonna play. Really immerse yourself in the type of music you want to play.