So do you EQ your whole mix based on the busiest part?

Will that not mean that there are "empty" bits when parts that occupy certain frequencies drop out of the mix, and nothing takes over that space?

I'm guessing no, but I'm curious about how it all works together. Had a cubase tutor who would talk about mixing bands, and he'd have to eq the lead singer so that solo'd she sounded like her head was in a bucket, but it was perfect in the mix. My confusion comes from the fact that if the song breaks down and she's singing a solo, she will still sound like her head is in a bucket.

The application of EQ, widening & reverb continue to confuse me...