Normally a singer should write his own line for he´s the one who must transport the song to the crowd, and he only can do that if he does what HE feels for this song and expresses it in his lyrics and melody.
But there are those musician who need YOU to write their part for they have difficulties in creating their own stuff. In that case (i can only emphasise this over and over again) you should have some basic knowledge about music theory - arrangement of triads (classical) is very helpfull. Also, u have to pay attention to the singer himself. you should write stuff that he can actually sing - it makes no sense to write soprano stuff for a baritone singer.
That again influences the arrangement of the song. The vocals must have enough space in a song to be heard! Heavy distorted guitars for example are at the same pitch (mid range) as "normal" vocals, so they might take up the space of the singer - this is the reason why many Heavy vocalists are singing extremely high.
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