Just reading your description, my hunch is that the director meant to indicate (by bringing Diana back at the very end of the song - ie. as a 'button' to the very last moment of the show) that *eventually* there would be a reconciliation.
If that's the case, the director is probably within his or her creative right.
If they had brought back Diana, for example, *before* the song and the conclusion of the story, that would be considered tampering with the ending, at least in terms of how these things are defined by the licensing house.
And to be honest, the end of NEXT TO NORMAL is essentially a tag -stepping outside of the perimeters of the time/space of the book. The song moves past the conclusion to the story and eventually becomes an after thought and a message of hope - a direct aside/plea/statement to the audience.
It's not a decision I would make as a director, but having Diana 'resolve' her relationship with the others simply by embracing them at the end of the song, isn't necessarily re-writing the linear story of what has transpired.
It's rather like bringing Angel back in the final moments of RENT. It's a symbolic gesture, obviously not to indicate anything beyond that. Probably where the director got the idea.
Updated On: 1/23/14 at 02:10 PM