No effects or prosthetics were created for Stu's missing tooth. Actor 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.
Lindsay Lohan was offered the part of the stripper, Jade, but turned it down. She did not like the script and thought the movie would bomb. Heather Graham was given the role instead. Lohan later admitted to regretting the decision.
During filming in Las Vegas, one of the Mercedes (a very beat up and distinctive one) was stolen from the lot where the vehicles were being kept. The next day the production was filming driving sequences and traffic was being held up by the local police. A production person noticed that a very distinctive Mercedes was part of the cars being held, the police were told, the driver was arrested and the car was recovered.
According to Celena Haas (Caesars Palace Director of Public Relations), the suite the characters stayed at was modeled after two of the resort's luxurious suites in its Forum Tower: a well-appointed penthouse and the so-called "Rain Man (1988) suite," located on the 68th and 69th floors of the Forum Tower.
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.
Ed Helms, Zach Galifianakis, and Bradley Cooper were all casual acquaintances before the movie was filmed, which Helms said he believed helped in establishing a rapport and chemistry amongst their characters. Helms credited Todd Phillips for "bringing together three guys who are really different, but really appreciate each others' humour and sensibilities." Helms also said the fact that the story of the three characters growing closer and bonding informed the friendship between the three actors: "As you spend 14 hours a day together for three months, you see a lot of sides of somebody. We went through the wringer together, and that shared experience really made us genuine buddies."
The plot was inspired by a real event that happened to Tripp Vinson, a producer and friend of executive producer Chris Bender. Vinson had gone missing from his own Las Vegas bachelor party, blacking out and waking up "in a strip club being threatened with a very, very large bill [he] was supposed to pay".
By depicting and celebrating Las Vegas as the "ultimate guys' getaway," the film had a major impact on Caesars Palace and Las Vegas. It was reported in 2013 that as of that year, guests continue to quote to Caesars staff two lines from the film's check-in scene: "Did Caesar live here?" and "Do you know if the hotel is pager friendly?" As a result of the film, Hangover-themed slot machines became popular at casinos throughout the Las Vegas Valley, the Caesars Palace gift shop sold tens of thousands of Hangover-related souvenirs, and the Las Vegas Convention and Visitors Authority received numerous inquiries from persons interested in recreating some of the film's most wild scenes, such as those involving a tiger.
During the scene where Phil, Stu, and Alan meet Chow in the desert, you can see their car pulling up in the reflection of Chow's sunglasses. This is a tribute to the scene in Casino (1995) where Nicky Santoro (Joe Pesci) meets Sam Rothstein (Robert De Niro) in the desert and you see the reflection of Santoro's car in Rothstein's sunglasses.
Regarding the explicit shots in the final photo slide show in which his character is seen receiving fellatio in an elevator, Zach Galifianakis confirmed that a prosthesis was used for the scene, and that he had been more embarrassed than anyone else during the creation of the shot. "You would think that I wouldn't be the one who was embarrassed; I was extremely embarrassed. I really didn't even want it in there. I offered Todd's assistant a lot of money to convince him to take it out of the movie. I did. But it made it in there."
The song that plays during the scene in which Neeco the tuxedo deliveryman tosses the guys' rented tuxedos into their speeding car is "Ride the Sky II" by Revolution Mother. Neeco is played by Mike Vallely, Revolution Mother's lead singer, and a pro skateboarder.
On the DVD commentary, Zach Galifianakis gives an explanation as to why Alan is not allowed to be within 200 feet of an elementary school - he innocently tried to play with some children which of course looked suspicious to parents. It was all just a misunderstanding.
The scenes involving animals were filmed mostly with trained animals. Trainers and safety equipment were digitally removed from the final version. Some prop animals were used, such as when the tiger was hidden under a sheet and being moved on a baggage cart. Such efforts were given an "Outstanding" rating by the American Humane Association for the monitoring and treatment of the animals.
Ed Helms said that the most difficult day of shooting was the scene when Mr. Chow rams his car and attacks the main characters, which Helms said required many takes and was very painful, such as when a few of the punches and kicks accidentally landed and when his knees and shins were hurt while being pulled out of a window.
Zach Galifianakis improvised the line "I didn't know they gave out rings at the Holocaust". The initial line had Alan making a sexually inappropriate comment about Stu's mom, but Zach thought up the new line for two reasons: he wanted to show that Alan was someone who knew almost nothing about the world, and that a successful joke that mentioned the Holocaust would establish that the film was going to be hilarious overall. Everyone at set loved the joke (Bradley Cooper appears to be disgusted by it, but he was actually following the script's directions for Phil to look at Alan in contempt, after the original line that was written out) and Todd Phillips said it would be in the film.
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."
The trivia items below may give away important plot points.
Ken Jeong improvised the nudity in the scene where he jumps out naked from the trunk. Originally his character was meant to have clothes on, but Ken suggested that his character be naked instead. Later, director Todd Phillips gave Ken a nudity waiver slip to sign in case Ken changed his mind on the nude scene.
According to Bradley Cooper, while giving a speech at a Toronto screening: "the people in Las Vegas never even noticed [a] movie was being filmed." He said that he had tiger scratches on his neck, all bloodied, and Justin Bartha was beet red, and nobody noticed, or even stopped to double-take.
In the MPAA green-band trailer, Ken Jeong is wearing digitally-added underpants when he jumps out of the trunk, and Bradley Cooper says "paging Dr. Douchebag" to Ed Helms instead of "paging Dr. Faggot".
The escalator scene in the casino is a tribute to Rain Man (1988). Alan and Phil have to win $80,000 in order to pay back Mr Chow. This is also the amount needed to win by Raymond and Charlie in Rain Man (1988).
When Phil, Alan, and Stu go to the clinic to consult with the doctor of Phil's admittance, the doctor doesn't provide much information which leads to Phil bribing him with a $100 while the doctor is washing his hands. The doctor agrees and tells Phil to slip the $100 in his coat pocket because he just sanitized. But in the next scene the doctor is seen holding a file, flipping through papers, and touching the front desk counter.