A dark subject would be the actual dialogue and subject matter. The plot is darker, more sinister, more serious.
While the dark ambiance is all the mood setters. Music and lighting chief among them.
The unfortunate thing with keeping the dark subject seperate from the dark ambiance is that it can be difficult to impossible to tell where one ends and the others begin. Having a character die instead of merely falling unconscious should be considered darker by most traditional assessments, yet it is not always. How dark the event seems is determined not just by what happens, but how it is treated by other characters. For example, had Cloud and company not been sad about Aeris' death, the event would have seemed a lot less dark than it was portrayed in the game, despite the actual event not changing. Would that be considered a change in subject, or in ambiance? It can be hard to say.
Regardless of that, the fact is, the best scene-writers always utilize both. Ambiance is can be every bit as important as the subject being dealt with. Though they will not always be used in a single unified way (every death scene is not accompanied by shadows and sad music), they are always planned out if you want the scene to have the right meaning. The wrong ambience can destroy a scene completely or totally change how it is portrayed, and to think that you can separate that from the subject is foolish.




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