Sylvester Stallone asked Chuck Norris to include a reference to The Chuck Norris facts (a popular internet meme) in the screenplay. Norris's wife Gena O'Kelly suggested him the one with the cobra. ("Chuck Norris was bitten by a cobra, and after five days of excruciating pain... the cobra died.")
Sylvester Stallone explained that his casting was looking particularly for actors who had not experienced recent hits: "I like using people that had a moment and then maybe have fallen on some hard times and give them another shot. I like those kinds of guys. Someone did it for me and I like to see if I can do it for them."
The film reveals that Gunnar Jensen earned an advanced degree in chemical engineering, then abandoned his science career to work as bouncer in order to impress a girl. Dolph Lundgren earned a chemistry degree from Washington State University in 1976, a chemical engineering degree from the Royal Institute of Technology in Stockholm in the early 1980s, and a Master's Degree in Chemical Engineering from the University of Sydney in 1982. Lundgren walked away from a science career to work as bodyguard for his then-girlfriend Grace Jones. While stuck in the mine, Gunnar refers to himself as a "Fulbright Scholar," which Lundgren achieved in real life.
While filming an action scene in Bulgaria, an explosion on a rubber boat killed one stuntman and left another in critical condition.