Andy gives Pam (in a kangaroo costume) a document to fax, and Erin joins her at the machine to see how she does it and also to get her neediness about Andy and her middling job performance all over Pam. Which is when we also learn that Andy put Erin in charge of the party. No wonder Andy's scared.
In the kitchen, Phyllis (in a more demure version of the sexy-lady costume), Stanley, and Oscar (wearing a suit, tie, lapel pin, and a nametag I can't read) are sitting around talking about a haunted walking tour when Pam drops an obvious hint that she wants to tell a creepy story of her own. After guilting Oscar into asking, Pam tells the story of how she used to work at one of the supposedly haunted bars on the tour when she was 22, and saw the reflection of an old man in black who wasn't there when she turned around. Jim has quietly entered and overheard, and he THs while laughing dismissively, "No, my wife does not believe in ghosts." The very idea.
In the break room, Jim privately asks Pam what she thinks she really saw, and she insists it was a ghost. Which she told him on their first date, but he thought she was bullshitting. And she appears to be pretty serious now.
In comes Robert California, dressed all in black and wearing Ray Bans, which Andy somehow identifies as a Jack Nicholson costume. Hell, I barely identified Andy's Jack Nicholson impression. Although I find myself recalling the time I went to work in costume as another James Spader character: Graham Dalton from Sex, Lies, and Videotape. Easiest costume ever, jeans and a black shirt. Robert's also brought along his son Bert, a pre-teen made up like a mini-zombie who wants to get on a computer to track a hurricane, because it's not like a kid who goes to his dad's work Halloween party in costume has anything else going on. Robert sets him down at Jim's empty desk as he notices the "pin the wart on the witch" game. Even Bert dismisses that as an immature activity, and Andy makes like the party is still getting set up. Over at Reception, Erin looks like she just got slapped, or maybe had a wart pinned on her. With a stapler.