Some writing tools are effective aids in helping to maintain the theme or premise of a film.
These involve the subtle use of devices like mirroring, and setup and payoff. Mirroring is the deliberate construction or establishment of elements that reflect another aspect of the story. Using the story described, we can build examples of layering and meaning. Perhaps if the scene in which Bob discovers the note from Sally comes in the middle of the script, it follows other similar events. Maybe Bob recently counseled friends of his who were splitting up.