Ok, few questions in there.
Like we were talking about with dialogue yesterday, a lot of that is subjective. However, you need to make sure that, above all else, the dialogue is intelligible.
Music plays a lot with emotion. It should swell and dim appropriately. If the dialogue is getting intense and the music starts fading out, you are sending a message to the viewer that what is about to happen is very important. If dialogue is getting more intense and the music starts swelling, you start preparing the viewer for forward motion in the story, or a big turning point, or action. Most of it is just getting it to "feel right".
If you have very loud music, yet want to make your dialogue cut through, you can achieve that with a basic eq. If you have a frequency analyzer, throw it over the dialogue tracks and see where the dialogue is sitting. The intelligibility cues usually sit in the 2k-8k range. Try pulling out a few db from the music that range (not the whole range, but a generous Q). Sweep the eq around a bit and figure out when the dialogue pops through.
Another way to do it is to put an eq over the dialogue. Try that same Q in the mid frequencies, except this time, boost it by 6db or more. This is going to sound terrible. Don't worry about it. Sweep around the 2k - 8k range, with a fairly narrow Q (not notch narrow, but not really wide either). At some point, you will hear the dialogue all of a sudden get much louder. Figure out what frequency that is, then pull a bit of that frequency out of the music. REMEMBER to turn off the eq on the dialogue after you figure out what frequency it is!
Hope this helps a bit!