Doing It for the Art: Nearly everyone in the film, but special credit goes to Benicio del Toro, who burned his arm with cigarettes to match the burns on Oscar Acosta's arm, and ended up getting blood poisoning. He also gained 40 pounds for the role. He tried to put on even more weight, but there wasn't time.
DVD Commentary: The Criterion Edition has several commentary tracks, including one by Hunter S. Thompson himself that makes the CE a must-have for Thompson fans. Throughout the film Hunter insults Terry Gilliam, screams randomly, audibly smokes weed, and tries to call cast members when he's bored. Yet for all his randomness, he remembers that Johnny Depp is using audio from the anti-drug law enforcement movie-in-a-movie on his answering machine and calls Depp's answering machine to prove it — and Hunter times it almost perfectly to sync up the film and the machine.
Mid-Development Genre Shift: It was originally just going to be a straight series of articles covering a motorcycle race in Las Vegas, before becoming the weird, wacky, genre-defining whatever-it-is that it became.
No Celebrities Were Harmed: Subverted by the first editions of the book, despite the best efforts of the publishers. When publication of the book was still being negotiated, the lawyers of the publishers tried to get Thompson to delete the references to his attorney engaging in criminal acts, since; even as depicted through an Expy, could have badly damaged his reputation and been considered libelous. Thompson reached out to Oscar Zeta Acosta; the real life model for the fictional Dr. Gonzo, and asked him to sign a release. Acosta however initially refused, not because he was concerned over libelous content, but because he took offense at the changes made to his character — he was a proud Chicano, and considered it an insult that the book described him as "a 300-pound Samoan". He would only sign the release on the condition that he be explicitly named as the basis for Gonzo on the book's cover. In other words, he insisted on taking full credit for the very criminal behavior the lawyers feared would harm and or offend him.
Benicio del Toro improvised the part in the beginning in the car when he licked the spilt cocaine off the suitcase.
Gary Busey 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.
Famed animator Ralph Bakshi wanted to animate a version of Fear and Loathing based on Ralph Steadman's illustrations. Hunter had given the film rights to one of his girlfriends, and couldn't dissuade her from making the live-action film instead.
According to Robert Rosen's biography of him, in early 1980, just before his recorded comeback, John Lennon read Fear and Loathing and became taken with the idea of playing Raoul Duke in a film version. Though it never eventuated, he referenced the novel in a draft of biographical matter (agreeing with its assessment of his song "Power to the People") and parodied the title twice in his posthumously published Skywriting by Word of Mouth.
During the early stages during the initial development hell to get the film made, Jack Nicholson and Marlon Brando were originally considered for Duke and Gonzo, and Nicholson was attached, but they 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 Duke, but he too grew too old. At one point John Cusack was almost cast, but then 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.