Stupidity: After Harkin says he isn't going to pour the scotch back into the bottle, Nick decides to chug the glass, spilling it down his chest in the process. It's an idiotic thing to do - if he'd taken two large gulps, the scotch would be gone (and he could relatively easily get some chewing gum/mouthwash to remove the smell from his breath). Instead, he decides to chug the scotch, spilling it down his shirt. Having clothes stinking of alcohol is much harder to hide in the office.
Stupidity: When Gail shoos Luke off the first day, in fact nobody accompanies him to the door and they discuss the evil plans when he's not even gone nor they even check he is, they just go ahead with the script. It's a rather odd dynamic, very rushed and movie logic-like. Or theater logic, since it's a typical 'exit stage left' moment.