I lived near Voiron and did the obligatory tour of the production facility (I already liked the liquor thanks to a friend (hey Iv!), and the place smelled delicious!) They do a good job maintaining the mystique of the Secret Recipe Known Only By A Handful™, which I suppose the monks tolerate and maintain to ensure the steady revenue stream. As one story goes, they were the "first internationally distributed" liquor when the French Foreign Legion loved the stuff and decided to always have some on their campaigns.
The Carthusian monks are an isolationist order, with intentionally limited contact with the outside, which dovetails well with Chartreuse mystique. There was a 2005 documentary, Into Great Silence [2], about life inside the monastery, whose filming was granted 18 years after it was first requested!
I'm sure Quentin Tarantino already has a good supply of the liquor ;)
If you're ever around the Alpes, try and find some genepi [3] liquor (not to be confused with absinthe! Made from same genus, different species,) which you'll find is 70% of the flavour of Chartreuse!
(dang, I miss Grenoble)
[1] https://fr.wikipedia.org/wiki/Grande_Chartreuse