Musical Entrances in Films
There should always be a noticeable reason why a film cue enters at a specific point, which usually is a cue that is being given by the movie itself. This could be a movement, a scene change, a cut, a line that is being said, a meaningful facial expression, the change of mood etc.
Starting music out of the blue without the audience pretty quickly noticing why will usually feel pretty disconnected and random, even if the purpose of the musical entrance becomes clear later on. So when you’re planning the musical entrances in your project, make sure that you place them where they actually feel like they have a reason to come in.
Additionally, depending on how “hard” the cue is given by the movie, it might be a good idea on softer cues (e.g. mood changes, slow movements etc.) to slightly move around the start of the music to find the point where the entrance feels placed best.