The humorous line, "No thanks, I've already had a wife", was improvised by George Clooney. Director Robert Rodriguez never intended it to be in the final cut, but after the studio included the line in a trailer, he felt obligated to include it in the film.
Salma Hayek has a real fear of snakes and had always refused to be near them. Naturally when she read the script, she knew her phobia would prevent her from taking the part. Robert Rodriguez conned her into thinking that Madonna was ready to nab the part instead so Hayek spent two months with therapists to overcome her fear.
Salma Hayek did not have a choreographer for her dance, because, according to her, you can't choreograph the live snake she wore around her neck. So director Robert Rodriguez just told her to feel the music and dance to it. Rodriguez used the same tactic with Jessica Alba in Sin City (2005).
Green blood was used for the vampires to get the movie past the censors.
Quentin Tarantino originally gave the script to make-up effects technician Robert Kurtzman to direct. When he couldn't commit, Tarantino showed the script to Robert Rodriguez, who eagerly signed on.
Lawrence Bender: The film's executive producer is sitting at the first booth at the diner where Jacob, Kate, and Scott are introduced.
Robert Rodriguez: [Trejo] Tattooed tough guy Danny Trejo also appears in Rodriguez's Desperado (1995), Spy Kids (2001), Spy Kids 2 - L'isola dei sogni perduti (2002), Spy Kids - Missione 3: Game Over (2003), C'era una volta in Messico (2003), Planet Terror (2007), Grindhouse (2007), Machete (2010), Spy Kids 4 - È tempo di eroi (2011), and Machete Kills (2013). In all these movies except for Once Upon a Time in Mexico (where he played "Cucuy"), his character had a "knife" name: Navajas in Desperado (navajas is Spanish for "folding knives", which his character used as throwing weapons), Razor Charlie, Isador "Machete" Cortez in the Spy Kids film franchise, and another Machete in Planet Terror/Grindhouse/Machete/Machete Kills.