The film was shot in Belize, Mexico, Guatemala, Costa Rica and Texas in the United States, over three weeks.
For about 90% of the filming the crew comprised seven people transported in one van: Ian Maclagan (sound operator), Jim Spencer (line producer), Verity Oswin the Mexican 'fixer', Edwards, a driver, and Able and McNairy, the stars.
As the low-budget production didn't run to a camera dolly, Edwards made do by sticking the camera out of the van window, cushioned on some bundled-up clothing.
As most of the extras were non-actors who were persuaded to be in the film, their action was improvised. "As a result of all this random behavior, the idea of scripting the film went out of the window. Instead I had a loose paragraph describing the scene with just the main points that had to be hit; how the actors carried this out was left up to them." Each night during the shooting period, the editor Colin Goudie and his assistant Justin Hall would download the day's footage so the memory sticks could be cleared and ready for the next day's filming. While new footage was being captured, the previously captured footage was being edited back at the hotel in which the production team was staying.
Back in the UK, Edwards had over 100 hours of unique, ad-libbed footage (rather than repeated takes of scripted scenes) to edit into a coherent film. Edwards did all the special effects himself using off-the-shelf Adobe software, ZBrush and Autodesk 3ds Max. The first assembly was over four hours long, but this was trimmed to 94 minutes after eight months of editing. Once the film was locked, Edwards had five months to create all 250 visual effects shots, a process he undertook in his bedroom. "[I was] churning out about two shots a day, which was fine until I got to the first creature shot. Then suddenly two months went by and I still hadn't finished a single creature shot; it turned out to be the hardest part of the whole process." Due to time constraints, the sound effects had to be produced before the special effects were undertaken.[10] Edwards claimed that the advances in computer technology in recent years made it possible for him to create the films visual effects on such a low budget; "You can go in the shop now and you can buy a laptop that's faster than the computers they made Jurassic Park on"