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 Get Carter (2000) and helped to pay part of Rourke's salary himself.
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 Cop Out (2010) at the time.
Sylvester Stallone sustained fourteen injuries making this movie, including breaking a tooth, rupturing his ankle, and getting a hairline fracture in his neck that required the surgical insertion of a metal plate. He also had bronchitis and shingles during the shoot. He later said he never fully recovered from the injuries, "It did such a number on my body, I've never been the same. Never."
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.)