It’s about the song, the performance, the arrangement and then the mix, in that order. Gear today is amazing, and amazingly affordable, so the greatest challenge to today’s recording engineer is in having something that’s worth recording in the first place; you get those things right, it’s hard for anybody further into the process to screw it up. (try though they may) The tricky thing is that the order of importance is not the same as the order in which you work on these things.
It starts with the song; if you don’t have a great song to begin with, you’re dead in the water. Beyond that, given that you have the song, you need a vision (or an auditory hallucination, really) of how it should sound, then you develop the arrangement in such a way that it’s easier or at least possible to perform and mix.
The arrangement is key to the performance-what’s the singer’s range, what are the highest and lowest melody notes to be sung, what key should it be in? does the pre-chorus need to precede every chorus or just the first? would modulating up a whole step for the last chorus increase the energy level?-and also to the mix-do we need to drop the drums out in the third verse where the singer gets up close on the mic and whispers?
Think about these things well before the recording session, then you have a roadmap for how you’re going to capture the performance in a way that enhances the song’s impact and makes it easier to mix. The mix is also a performance, so it makes sense to plan out and rehearse the mix moves ahead of time as well.