You can see in this trailer Nathan doing something weird with his tongue and his tooth. In an interview, Nathan explained that when he's trying not to laugh or break character, he presses his tongue against his molar really hard. So him pressing his tongue against his molar really hard reveals that Nathan knows that Elizabeth Holmes is a lying manipulator, and it also indicates that Nathan is also trying to manipulate her and is in character. So then the way Nathan acts in the documentary is an act, he's conning a con artist.