While filming took place in the actual Mercer house, production could not find an insurance company that would underwrite the project given the extensive value of the antiques. All of the items seen in the movie are therefore replicas with the originals stowed in storage during filming.
For the Christmas party scene at Mercer house, they actually invited a large number of people who had previously been at James Arthur Williams' parties, including several members of his family. These extras praised the movie party because, not only was the atmosphere correct, but Kevin Spacey perfectly played James and his style of schmoozing at parties.
There is a reference in the book, published in 1994, where someone mentions at a party about their suspicions regarding a relative's cremation. The ashes were tested and found to be wood ash. Eight years later the Tri-State Crematory was proved to have been inappropriately disposing of remains and giving wood ash instead of ash remains to the relatives of deceased persons.