What is Fache's actual role in the book and how is it revealed in Chapters 101 through 104?

Fache's actual role is revealed in Chapters 101 through 104, when Langdon is able to extract the last clue from the keystone and pocket it, without Teabing seeing him. He knew Teabing would drop everything to protect the keystone, so he throws it up in the air to break it and ends up with the gun aimed at Teabing. It is discovered that Bezu Fache was actually one of the good guys when he comes to Langdon's and Sophie's rescue and arrests Teabing.