"singing" Singing

DarkNoise

Grotesque Butchery Rabbit
Alright, as an alternate to the growel / scream posts I've participated in recently, I have a question on actual singing. For the songs and bands that actually have singing parts that rely on the harmony and melody of vocals, what have you noticed commonly between em (going a lot on Opeth's singing here). Is there a specific mode to sing in, do you follow the same principles with vocals as you do when trying to harmonize two guitar parts (ie in metal using minor thirds on guitars, so you sing minor thirds as well)? I'm aware that every singer has his or her own style, but the same can be said for guitarists.

Anyway, simple way of putting it - what patterns do you notice for singing in metal?