No effects or prosthetics were created for Stu's missing tooth. Ed Helms never had an adult incisor grow, and his fake incisor was taken out for the parts of filming where Stu's tooth is missing.
Ken Jeong's wife Tran was battling breast cancer and undergoing chemotherapy treatment, which was difficult for the family as Ken was shooting the film in Las Vegas. Bradley Cooper drove Ken to Los Angeles so he could be there for his wife. Cooper invited the couple to spend the holidays with him and his family. Ken said, "It got me through the most difficult period of my life." Tran was declared cancer-free after the film wrapped.
Mike Tyson revealed that he appeared in the film to fund his drug habit, and that he was high on cocaine when he filmed his scenes. Tyson later said that working on the film convinced him to change his lifestyle.
When the cop car pulls up in front of Caesar's Palace, two real employees are having an argument in the background about the filming of the movie.
Ed Helms' and Zach Galifianakis' songs ("Stu's Song" and "Three Best Friends", respectively) were improvised.
Zach Galifianakis: [Chuck E. Cheese] Alan makes a joke about him not being allowed near schools or a Chuck E. Cheese, a direct reference to one of his staple jokes in his stand up: "When you look like I do, it's hard to get a table for one at Chuck E. Cheese."
Lucinda Jenney: At the casino, playing the same character she played in Rain Man - L'uomo della pioggia (1988).
Todd Phillips: [band] The foul-mouthed band featured near the end of the movie is the "Dan Band", the same band from Frank's wedding in the movie Old School (2003) and Vince Vaughn's daughters' Bat Mitzvah in Starsky & Hutch (2004).