Captain Fache's nickname's apt, and his "unblinking severity" makes Langdon a bit uneasy as he enters the dimly lit museum.
There are French agents all over the museum. The regular security guards are being questioned. It's obvious Fache is running a tight ship.
Fache questions Langdon on their way to the scene of the crime, paying particular attention to the meeting they had planned for that evening. He thinks it's odd that Langdon didn't know why Saunière wanted to meet with him.
Langdon hadn't asked why Saunière had contacted him. He was just thrilled to be able to meet him, because he was a revered expert on something Langdon was writing a book about: the iconography of goddess worship.
What's odd is that no one knows about Langdon's latest manuscript, because Symbols of the Lost Sacred Feminine (his new book) proposed some very unconventional interpretations of established religious iconography that would be very controversial in the right circles. So for now just he and his editor know about it.
Langdon's freaking out because Fache makes him take an elevator to the gallery where they found Saunière. He's really, really claustrophobic due to a childhood trauma in an abandoned well. (Where's Lassie when you need her?)
In his desperation to not panic, Langdon fixates on the captain's tie clip, which is a silver crucifix with thirteen embedded pieces of black onyx. Known as a crux gemmata, it's a Christian ideogram for Christ and His twelve apostles.
Exiting the elevator, Langdon's stunned by how different the gallery looks in the dim, red, service lighting used at night.
They walk past Saunière's office, where the DCPJ has erected their makeshift command post. Fache tells the agents that he and Langdon are not to be disturbed at any cost.
They proceed to the Grand Gallery, where they have to slide under an enormous steel grate—the containment security Saunière activated by pulling down the painting.