A common question I am asked is, “With which instrument should I start mixing?”
Every hit song needs a 'hook'. It is the feature that compels people to buy. The hook may be a beautifully written vocal line, beautifully sung. But take a listen to tracks from the current Hot 100 chart and you will find that this is not always so. Many times the hook is an instrumental phrase or harmonic progression that occurs on several occasions during the song. It might be a real instrument, or a synthesized sound. It might be a percussive element. Whatever it is, it attracts the ear, and attracts sales.
Occasionally also, a song might have a certain instrument that is of major importance in the arrangement and production. This may be to such an extent that it is vital that this instrument is shown off to its best in the mix, while the vocal may need only a standard professional treatment.
So although this will not apply to every song, on many occasions there is a certain instrument that gives a song its magic. And this instrument must be presented to the listener as well as the mix engineer is capable of.
If this is so, then it makes sense to start mixing with this instrument being the only one in the monitors, even if it doesn't play all the way through the song. You need to work hard to make the instrument sound amazing, or amazingly interesting (notice the subtle difference there).
By 'work hard', I mean use all of the processes you have available to get the best from the instrument – fader, pan, EQ, compression, reverb and any other effects you feel that you need. You may also need to do some editing – for example a beautifully-played acoustic guitar part might have a few finger squeaks.
If a particular instrument really is important in a mix, then it makes sense to spend time on it. As much time as necessary to get the absolute best from it. Some of that time will be spent in experimentation, some in consideration and detailed listening. Once you have achieved the best you can from the instrument, the other instruments and vocals might only require a standard, professional treatment, which can be done more quickly as you are applying techniques and achieving sounds that are already comfortably within your range of experience.
My comments on drums and vocals still apply, but sometimes you may need to decide to put the bulk of your time, energy and effort into a particular single instrument.