Mixing Dialogue Underscore
Scoring dialogue scenes comes with a few specific requirements, all of which put emphasize on the necessity to keep the speaking voices understandable in the final mix. In writing, a general rule of thumb is to treat the speakers as if they were instrumental solos and write accordingly “around them”.
However, the mix of the music can also be a factor in this equation. In most cases it will help to try mixing it as “spacey” as possible in the critical frequency range (low mids). In spite of the writing doing a lot to keep this frequency range clear, some room resonances and higher harmonics of low instruments might still occupy this space quite heavily calling for active counter measures.
In such cases it helps during music mix to once in a while hear it with the actual dialogue and check against it. This can even go as far as having a slight feeling of “something missing” in the pure music mix which will be no problem in the movie as this frequency range will be covered by the voice anyway.
Of course, all measures you take should not cripple the music for the sake of making it more transparent so keep the adjustments you do within “realistic” proportions.