Hi There!!!!!!!!!
Okay, lesson #1: Work closely with the SSE/SD (Supervising Sound Editor/Sound Designer) in conjunction, of course, with the director. Just as one example - there is a section of a chase scene where the director wants the footsteps of a character to be very prominent. If the score is heavy on percussion in that section it will be very difficult for the rerecording mixer(s) to achieve a suitable balance. (As you can probably guess I do audio post.) So discussions & cooperation between you and the audio post folks will greatly enhance the project.
Lesson #2: Try to avoid frequencies/instrumentation that occupy the same range during dialog. Admittedly, this can be difficult if the dialog is between say, James Earl Jones and Jennifer Tilly.
Lesson #3: It is the directors project. His/her word is final. If you can't handle that then look for another line of work. This is not to say that you shouldn't "stick to your guns" when there is a creative disagreement; I have argued vociferously, albeit very politely, for or against _____________ (fill in the blank) with a number of directors. Sometimes they agree, sometimes they over-rule, sometimes we both collaborate on a different creative direction. But in the end it is the directors project and their creative vision; you/we are merely there to creatively/artistically bring that vision to life.
You have a whole lot to learn - working with directors, working with battling creative partners (producer vs. director, co-director vs. co-director, etc.), working with editors and audio post people, dealing with time code, and many, many more new experiences.
We're here if you need us.
Peace,
Uncle Bob