The SONG
Before you start your mix, you should have a good understanding of WHAT you are doing. Sure, you can make "product" that has all of the typical characteristics of other variations of the same type of "product". In other words, you can make a mix that sounds and feels exactly like every other song in the same style. You can make a rock mix that has a heavy snare and pumping guitars, you can make a dance mix with a big kick / bass, and you can call it a day.
But what are you REALLY supposed to be doing? Aren't you there to mix the SONG? Aren't you there to understand what the artist is trying to SAY and make that statement more compelling, expressive and more easily communicated to listeners?
If you do not understand the SONG, then you are only making PRODUCT. You may make amazing product, but ultimately it will be easily replaced by the next amazing product that comes along.
I am much more concerned with mixing SONGS. I cannot mix anything that I do not feel.
When I begin a mix, I start by finding out what the artist wants. If the artist is with me, I will ask what the song is supposed to be about (beyond the lyrics). If the artist is not with me, I usually request "too much information" about the song.
Once I get past finding out what the artist wants, I start working with the tracks. Sometimes tracks will contain hidden intentions that the artist may not have consciously meant to create. I try to find the "song". To do so, I start by listening with all tracks playing. Then I listen to various combinations of fewer and fewer tracks, trying to find a bare version of what is being said. This combination frequently includes vocals and a harmonic instrument such as piano, guitar or bass.
When I can strip the song down to the vocal and just enough instrumentation to support the vocal without overpowering it, I often find emotional expression that may have been hidden with all the tracks playing.
THAT'S the song.
Sometimes it comes down to vocals and drums rather than vocals and harmonic instruments. Sometimes I need to process some instruments to create a specific space that allows the song to speak clearly. A great example of processing that helps a song to speak (or shout) more clearly is the delayed room sound on the kick drum of Robert Palmer's "Hyperactive" from the Riptide album. The amazing ET Thorngren created a rolling rhythm that provided a much better foundation than the original part would have. While I am no ET Thorngren, I have often setup elaborate effects in my attempts to find enough of the song so I could begin actually mixing it.
So for all the assistant engineers who asked me why I was so mad only to hear me yell out "I can't find the song!" this is why I was so frustrated...
...if I can't find the SONG before I mix, I will only end up mixing PRODUCT. Product is boring and predictable and does not speak to me artistically. SONGS will move me to rage, joy or tears.
I keep saying that as a mixing engineer my job is to create an illusion. I know when it is right when I am believing the illusion as I am creating it, and that belief causes me to do things to make the illusion seem more real.
And yes, I have been moved to tears while mixing, and have moved producers (who have lived with the material for months) to tears as well. If this has never happened to you, then maybe you have been mixing "product" and not "songs". Product can be great, but songs will rip your heart out.