Hunter S. Thompson had previously been portrayed by Johnny Depp's Ed Wood (1994) co-star Bill Murray in Where The Buffalo Roam (1980). Prior to filming, Murray called Depp with the advice "Be careful, or you'll find yourself ten years from now still doing him...Make sure your next role is some drastically different guy."
Near the beginning of the movie, while Dr. Gonzo and Raoul are driving down the highway, there is an accident involving many cars. There is an ambulance about to put a person that is covered with a white sheet in it. If you look on the white sheet, there is a smiley face in blood on it.
The t-shirt that the hitchhiker (Tobey Maguire) wears, has a Ralph Steadman picture on it. Ralph Steadman did the original illustrations for the book, and the typeface of the credits is based on his handwriting.
Near end of this film, Duke takes too much "andrenichrome" and has a nasty experience. Andrenichrome was a substance that Hunter S. Thompson made up for the book when he originally wrote it, and was kept in the script by Terry Gilliam. The name itself wasn't new - Adrenochrome is an oxidation product of adrenaline, while Adrenochrome semicarbazone, also known as carbazochrome, is used as a medicinal drug to reduce capillary bleeding - however, neither compound is a hallucinogenic drug as portrayed in the book and film. After showing a rough cut of the film to a test audience, Gilliam was approached by a group of young men, one of which complimented him on the film in general, but said that his favorite scene was the andrenichrome scene. He said that he had used the drug, and that Gilliam had captured the effects perfectly. Gilliam didn't have the heart to tell the kid that it was made up, and went along with his story.
Prior to filming, Johnny Depp swapped his car for Hunter S. Thompson's red Chevrolet convertible, and spent weekends driving it around California in preparation for the role. Meanwhile, Thompson spent that period in Depp's car with a woman named Heidi, writing an essay called "Fear And Loathing In Hollywood: My Doomed Love At The Taco Stand" that was partially published in Time Magazine, along with a new Ralph Steadman drawing of a gargoyle-like Dr. Gonzo.
According to Terry Gilliam's commentary on the Criterion Collection DVD, at the beginning of the movie when they stop on the side of the road after Duke starts seeing bats and wants Gonzo to drive, you can see a strange looking cactus in the background. It was designed by Ralph Steadman and appears many times in the background of the movie in various scenes. Gilliam complained of having to lug it around wherever they went.
While Hunter S. Thompson developed a strong friendship with Johnny Depp and heartily approved of his performance, he once said that if he ever saw anyone acting the way Depp does in the film, he would probably hit them with a chair.
Much of the clothing (shirts, hats) worn by Johnny Depp in the movie were actual pieces of clothing that the real Hunter S. Thompson wore in the '70s. Thompson himself let Depp borrow them for the movie, after Depp spent four months with Thompson learning his mannerisms and proper vocal inflection for the role.
In the scene where Benicio Del Toro stops the car and has an "attack" with Tobey Maguire in the car, Maguire's hair seems different. According to Terry Gilliam's commentary on the Criterion Collection DVD, they shot this scene a few months after the scene where they first picked him up, and could not get Maguire to shave his head for the wig. It would have cost fifteen thousand dollars extra to put that in his contract initially, but they opted not to, because the movie was already becoming over budget. They ended up spending well over that using a bald cap and make-up effects, as well as using computer editing to erase the line on his forehead.
Gary Busey (Highway Patrolman) improvised the "Give me a kiss" line. The producers and Hunter S. Thompson were initially horrified by it, but Terry Gilliam thought it was funny, and left it in the final cut. Thompson said that after a few more viewings, he found the line quite funny.
Terry Gilliam took over as director after Alex Cox (Sid and Nancy (1986)) left the picture due to creative differences. Gilliam re-wrote the entire screenplay in a matter of a few days to fit his unique creative vision and style, while staying true to Hunter S. Thompson's writings.
Duke's tribute to Dr. Gonzo - "There he goes. One of God's own prototypes. Some kind of high-powered mutant never even considered for mass production. Too weird to live, and too rare to die" - is taken from the foreword of "Autobiography of a Brown Buffalo," the semi-autobiographical novel written by "Oscar Zeta Acosta". Zeta Acosta was the famous "Chicano lawyer" and friend of Hunter S. Thompson, whose notorious party binges served as the model for Dr. Gonzo.
During the scene in which Duke is tripping on adrenichrome, he mutters, "La llama es un quadrupedo." This is a quote from a sketch in Monty Python's Flying Circus (1969). Subtitles on some versions of the film simply say "Duke mumbling incoherently".
In a scene cut from the movie, Duke and Gonzo tell a D.A. from Atlanta about a rather gruesome incident which happened at a McDonald's. In the final cut, during the check-in scene at the Mint Hotel, a man in a cowboy hat tells the exact same story to someone over a payphone.
According to Terry Gilliam's commentary on the Criterion Collection DVD, in the scene where Raoul and Gonzo raise havoc at the Debbie Reynolds concert, the voice heard in that scene that is supposed to be Reynolds singing, actually is Reynolds. Gilliam was friends with Carrie Fisher, Reynolds' daughter, who spoke to her mother about recording a couple lines for the movie, and Reynolds agreed.
The scene in which Raoul Duke calls his attorney from Baker, California, is partially filmed backwards. In the background, smoke can be seen coming back into a fire, and Duke bangs in reverse on the side of the phone booth.
When Raoul Duke is calling his attorney about a new assignment, there is a poster on the back wall of Dr. Gonzo's office. It has a two-thumbed fist clutching a peyote button inside a Sheriff's star. This is actually a political poster from Hunter S. Thompson's campaign for Sheriff of Aspen. He ran on the Freak Power party ticket, a political party he made up himself. The "gonzo fist" symbol can also be seen in the bathtub scene, written on the wall behind Duke in shaving cream.
When Raoul Duke goes out to gamble, and comes back to find Dr. Gonzo tripping out on acid in the bathtub, there is the infamous Gonzo Fist drawn with shaving cream on the pink bathroom wall behind him.
Animator and filmmaker Ralph Bakshi tried to convince Producer Laila Nabulsi to let him do this movie as an animated movie, done in the style of Ralph Steadman's illustrations for the book. Bakshi is quoted as saying: "Hunter had given the rights to a girlfriend of his. I spent three days with her trying to talk her into me animating it - she wanted to make a live-action of it - I kept telling her that a live-action would look like a bad cartoon, but an animated version would be a great one. She had a tremendous disdain for animators, because it wasn't considered the top of Hollywood. Hunter also could not make her change her mind. So she made the pic with Johnny Depp (who is a great actor) and got the film I told her she would get - it would have been more real in a cartoon, using Steadman's drawings."
The Bazooko Circus Casino was modeled closely after the Circus Circus Casino in Las Vegas, which was mentioned in the original book. The real casino refused to have anything to do with the film, and even forbade the use of its name. The casino sign shown in the film replaces Circus Circus' neon clown's pinwheel with a mallet, and the interior shots were partially filmed in the (now closed) Boardwalk Casino in Las Vegas. The carousel-themed "Merry-Go-Round" bar in the real Circus Circus (called the "Horse-A-Round" bar) revolves in the opposite direction as the one depicted in the film.
During the initial development hell to get the film made, Jack Nicholson and Marlon Brando were originally considered for the roles of Duke and Gonzo, and Nicholson was attached, but he, and Brando, both grew too old. Afterward, Dan Aykroyd and John Belushi were considered for the duo, but that fell apart when Belushi died. John Malkovich was later considered for the role of Duke, but he too grew too old. At one point, John Cusack was almost cast, but then Hunter S. Thompson met Johnny Depp, and was convinced no one else could play him. Cusack had previously directed the play version of "Fear and Loathing", with his brother playing Duke.
Laraine Newman (the Frog-Eyed Woman) and Harry Dean Stanton (the Judge) both appeared in the 1996 spoken word adaptation CD of Fear and Loathing in Las Vegas. Stanton was the narrator, while Newman played several small roles.
During the montage at the beginning of the film, (where Raoul and Dr Gonzo drive around collecting things for the trip), there is a glimpse of a bunch of people packing things onto a psychedelically-painted school-bus. This is most likely a reference to Ken Kesey and his band of Merry Pranksters, who also drove around in a psychedelically-painted school-bus.
The coconut-smashing scene towards the end of the film was not originally in the book. Hunter S. Thompson wrote the scene when he wrote Fear and Loathing, and then omitted it. Terry Gilliam inserted it back into the story for this movie.
During the scene when Duke first arrives at the dirt bike race, it is suppose to be dawn, but you can tell from the lighting it certainly isn't. According to Terry Gilliam's commentary on the Criterion Collection DVD, they had the shot set up perfectly with the sun just rising, but when Johnny Depp tried to start the car, it was dead. They figured out soon enough that the driver forgot to fill the tank. Due to budget and time constraints, the shot had to be redone later that day.
Terry Gilliam, Tony Grisoni, Alex Cox, and Tod Davies are credited for the film's screenplay. Gilliam and Grisoni adapted the novel by Hunter S. Thompson, and used their script for the film. However, a nasty arbitration wrangle at the Writers Guild of America found that Cox and Davies had written an earlier screenplay, that had also adapted Thompson's novel, with several similarities to Gilliam and Grisoni's screenplay, and should therefore receive credit for the finished work, instead of Gilliam and Grisoni (who the Guild claimed did not contribute enough new material to their script). After Gilliam appealed this decision, a shared credit for all four of the writer was permitted. To this date, both Alex Cox and Terry Gilliam claim to have been wronged in this.
Hunter S. Thompson's tribute to Oscar as he boards the plane was actually taken from "The Banshee Screams for Buffalo Meat: Fear and Loathing in the Graveyard of the Weird," and was later added to the forward of Acosta's autobiography. The forward is little more than a quote, while the original article is a fascinating eulogy on the mysterious disappearance of his friend Oscar. The article is reprinted in its entirety in "The Great Shark Hunt", Hunter S. Thompson.
Bruce Robinson, director of Withnail & I (1987), was asked by Johnny Depp to write and direct this movie. Robinson turned the offer down, saying he couldn't see "how you could get that one on the screen." Conversely, Robinson later accepted an offer to write and direct a screen adaptation of Thompson's novel The Rum Diary (2011).
In retaliation for the wrangle that Terry Gilliam faced against the WGA, on May 22, 1998, during a book signing, he burned his WGA card in front of the public. That led to Gilliam and Tony Grisoni being given the dual writing credit.
In the check-in scene, towards the beginning of the movie, a man is heard describing a murder, where the victim was decapitated and had their pineal glad removed. In a later scene, Duke consumes adrenichrome, and it is stated that the drug can only be obtained from the pineal glad of the human brain.