Instead of using CGI, Darren Aronofsky chose to do the special effects for the film by using micro-photography of chemical reactions on tiny petri dishes. He has said that CGI would take away from the timelessness of the film and that he wants the film to stand the test of time.
In early 2002, writer/director Darren Aronofsky cast Brad Pitt and Cate Blanchett in the central leads of Tom and Izzi with a budget of $75 million. During pre-production, Pitt and Aronofsky were having major creative differences, so Pitt left to film Troy (2004) instead and the film was shut down, and the sets and props built in Australia were auctioned off. In early 2004, with a smaller budget of $35 million, Aronofsky cast Hugh Jackman as Tom and Rachel Weisz replaced Blanchett as Izzi. Warner Bros., who had invested 20 million dollars in the canceled version, agreed to finance the new, cheaper version.
Of the 70 extras cast as Mayan warriors, 20 were actual Mayans flown in from Guatemala. Fernando Hernandez, who played the Lord of Xibalba, was the only one who could speak English. Before shooting at the Mayan pyramid, the Mayan actors blessed the set.
When the first version of the script was shelved and production was halted Darren Aronofsky rewrote the script and re-envisioned the film. The first version of the script was turned into a graphic novel illustrated by Kent Williams and released as a soft-cover and limited edition hardback book. The final version of the film, with its reduced budget and new leading cast, made extensive use of unique non-CGI special effects to save on production costs and give the picture a more "organic" feel.
A piano and vocal interpretation of Clint Mansell's "Stay with Me" theme was originally recorded for the end credits, with Justin Skomarovsky on piano and Antony Hegarty (of the group Antony and the Johnsons) on vocal. Director Darren Aronofsky, though satisfied with the piano arrangement, didn't feel that Antony's lyrics consummately reflected the film's message and narrative. Ultimately, Aronofsky discarded the song and opted instead to record Skomarovsky's piano arrangement with veteran Hollywood pianist Randy Kerber.
The group of stars that appear in the movie is the Orion constellation, and the Xibalba Nebula is known in astronomy jargon as Messier 42, M42, NGC 1976 or Orion Nebula, located at 1270 light-years approximately from the Earth.