Childermass is a master of deception and disguise, and it is for that reason and that reason alone that he manages to keep a straight face when confronted with the idea of a threesome with Norrell. He does however shoot her an inscrutable look over his shoulder, the exact meaning of which entirely indecipherable -- and purposely so.

"He is not one for sharing in anything that he does," Childermass drawls in return, turning back to lead her into the house, shutting the door behind her, and stepping forward to continue through more tastefully expensive rooms. "You need not worry about me."

No, Childermass is not the one that she need worry about, as he directs Nancy through the rooms and opens the door to the study for her, ushering her in and closing the door behind her. Straight into the mouth of the beast.