Something's unclear in the plot
I finished watching Vidocq few minutes ago. It was on TV and I missed the begginning so maybe there was the answer to my question.
So here it goes:
If the writer and alchemist was the same person, who was following the writer while he was investigating in this evil district? We could see a shadow behind him a few times. This is pretty irritating because, everybody watching the film would think that it was the alchemist's shadow - so it would completely exclude the writer being the main villain... I understand that this trick made everyone astonished when the identity of the murderer was revealed, but it wasn't explained in the movie since what I know.