"an inner mystery"
The first scene in which we realize the collusion between Virgil and Billy has some very ironic lines, after Billy protested that he was regularly satisfied with his cut of the profits, that he just wished his friend would recognize his "great artistic talent"-- to which Virgil replies: "The love of art and knowing how to hold a brush doesn't make a man an artist; you need an inner mystery, and that, my dear Billy, you've never possessed."
I just love the dramatic irony that Virgil doesn't realize yet just how deep, dark, and wickedly manipulative his friend's "inner mystery" is.