Café de Nuit

I will be running this module soon and I want to know if any body has any advice on how to handle the pc are secretly replaced by carionette part.
Should I tell the pc who get replaced or should I reserve the surprise at the end?

I have run a similar situation and what I did was I let the PC slowly figure it out for himself. He passed out (the possession) and when he woke up he was in a closed box which he thought was a closet. Then when he reached up to open the box he found that his hand was made out of wood.

save it till the end. it is always better. seriously.