do you think we need to see the ghost girl leave the other girl's body - and do you think she needs to be blue? I suspect your audience will understand the significance of the double soon enough, and aren't you giving the game away too soon by acknowledging so explicitly that the other girl is a ghost? Perhaps if she was just 'there' - standing opposite the other girl in a non-showy way - it would remain more ambiguous and classy? What do you think?
Well I was just going to have her just appear, as in the girl would turn around to see her in the doorway or something, kind of mirroring what the non-ghost girl did when she entered the room earlier on. Maybe that's too dramatic...Maybe if she was just 'there' looks of confusion would be seen and as the 'ghost' goes to get the music box, instead of walking around the girl she just goes straight through her? or is none of that really needed?
I think your original instincts are right re. just having her 'there' - why not put together a really quick 'thumbnail' storyboard and see how the alternate ideas hang together - time to get drawing, as it's clear the foundations of your story are becoming more and more settled - maybe just draw these more complex sequences out and test them that way?