Since I've worked in film and television for so long, I've acquired the ability to let the version of the characters that lives in my mind make way for the living, breathing humans who are going to play them on screen. If you cast it right - and casting is about 80% of directing - they will eventually replace or exceed the imaginary image.
Singing harmony is not the same as singing a part in a choral group, where you know you're going to have to hit this note and then that note. There are nuances that change every day. Maybe today you have a slight cold or voice fatigue, or you've done something and there's a slight difference in your breathing.