Mickey Rourke said he did this film as a favor to Sylvester Stallone, who years earlier, when Rourke was in a career slump, offered him a major role in El Implacable (2000) and helped to pay part of Rourke's salary himself.
Sylvester Stallone sustained fourteen injuries while making this movie, including breaking a tooth, rupturing his ankle, and, during an intense fight scene with Steve Austin, getting a hairline fracture in his neck that required seven major surgeries, spinal fusions, and the surgical insertion of a metal plate. While the neck fracture was the initial injury, it has had a debilitating effect on Stallone's overall spinal health since then, leading to persistent back problems. He later said: "I never recovered from 'Expendables 1.' After that film, it was never physically the same. I've warned people: Don't do your own stunts." He also had bronchitis and shingles during the shoot.
In the audio commentary Sylvester Stallone reveals that in the final battle, Steve Austin almost lost his leg due to an explosion that went off next to his foot. What saved him was the bag he was holding took the bulk of the blast. The bag acted as a shield when the explosion happened. (You can see this happen at the 1:24:05 mark in the movie.)
Sylvester Stallone states on the commentary that Church's line, "You guys aren't gonna start sucking each other's dicks are ya," was ad-libbed by Bruce Willis. He at first had reservations about the line but decided keep it in the movie.
The role of Mr. Church, the man who hires the Expendables, was very difficult to cast. It was initially offered to Arnold Schwarzenegger, who declined in favor of a smaller cameo due to his political career. The role was then offered to Kurt Russell, who declined as he was not interested in "ensemble acting at the moment." Sylvester Stallone spent several months after principal photography determined to find a big action name for the part. The role was finally taken by Stallone's friend and fellow former "Planet Hollywood" co-owner Bruce Willis, who was busy filming Dos inútiles en patrulla (2010) at the time.