Okay, I think this would be a good song to use to illustrate some of what I am talking about
:
Carcass - Corporal Jigsore Quandary
The structure of this song makes it easier to pick apart the different things that are going on - in the intro you get the drums playing a certain thing, then one guitar comes in playing the same rhythm, but since guitars can play notes and chords and stuff, you get a bit of melody involved. Then the second guitar comes in playing a similar melodic line, but without the rhythmic complexities. (Sadly the song is a bit too repetitive and gets boring, but it's tough to beat the first minute of it.)
This sort of overlay between the instruments is what makes the music interesting to me. Sometimes they will all be playing the exact same rhythm... sometimes they will play similar stuff that complements the other.
Something else that you get a lot of in extreme metal is that the guitar will often be hitting the same note repeatedly - sometimes this is part of the melody and sometimes it is not so you have to be able to sort of what is going on. In most types of music the guitar is just going to be playing chords - in very fast metal it is not possible to play chords at that speed so instead you may just get a single note that represents the root of the chord.
So for example in a song like this one:
Amon Amarth - The Pursuit of Vikings the guitars at the beginning keep playing a single note in a rhythm which is eventually picked up by the drums, while the melodic line is then woven in starting at 0:39. In passages like this you can have a single instrument that is providing both the bassline and the melody. I would not consider Amon Amarth to be an exceptional band but in this song it is really easy to hear what I am talking about. For more of a challenge you could try picking apart something such as
Death - Flattening of Emotions.