The flame breath was inspired by the bombardier beetles, small carabid coleopters. Their peculiar defense mechanism involves the ejection from their abdomen of two different chemicals, which react and produce a hot noxious spray. This was the actual base for the dragons' breath, as explained by Dan DeLeeuw: "our art director, Mike Meaker, had seen an African beetle with a similar defense mechanism. It sprayed a chemical out its rear and made sparks to ignite the stream. So our idea was that there are two glands at the back of the dragon's throat, producing chemicals. The dragon spits out the liquids with muscles constricting the glands. When the two streams cross 15 or 20 feet in front of the dragon they naturally combust." Before actually spitting fire, the dragons pull back and shake their head -- then thrust it forward violently. This behaviour was once again based on spitting cobras.
Christian Bale originally planned to be really skinny and scrawny for the film, figuring that in a post-apocalyptic landscape, food wouldn't be so readily available so most people would lose drastic amounts of weight. But when he showed up for filming and saw how bulked up Matthew McConaughey had become, he had to change his mind and hit the gym. Considering the two have an intensive fight scene in the film, Bale reckoned it would look more evenly matched and therefore more realistic if the two men were of similar build.
It was filmed in Ireland, in the Wicklow Mountains. They were only allowed to film there under the condition that they clean up entirely after themselves, without damaging the landscape.
In an interview with the AV Club in 2015, Alexander Siddig (Ajay) said, "The only thing I remember about that was the first day. The first A.D. came into the trailer where we were all having our makeup and shit done, and he was, like 'Guys, I need your attention, please.' And we were, like 'Yeah?' And he said, 'Um, Mr. McConaughey's gonna arrive on set in about 15 minutes, and I have to give you a directive - which comes from the producers - that you are not to call him 'Matthew' or 'Mr. McConaughey' or anything to do with his real life. You must call him Van Zan.' Van Zan was his character name. 'And even if you meet him outside in the road, even if you meet him out in town in Dublin,' where we were shooting this movie, 'You must call him Van Zan.' And that is exactly what I remember about that movie, because as that first A.D. left the building, I shouted - rather lamely - 'And he's got to call me Elvis!' But he didn't call me Elvis. In fact, he didn't call me anything!"
Matthew McConaughey says in his 2020 book Greenlights he shaved his hair prior to filming as he was losing his hair