Townsend, a movie actor himself, told AP Radio that Theron was doing a back-flip somersault while wearing platform shoes when she slipped and hurt her neck.
"The slipped disc went almost into the spinal cord," he says. "She's fine, but could've been in a lot of trouble."
The film, which was being shot in Berlin (search), has halted production for several weeks. When news of the accident was first reported on August 31, a publicist for the Paramount movie did not specify the extent of Theron's injury, but said it happened during a wire-hanging action sequence, and that the part was "a very physically demanding role."
Townsend says he expects her to be laid up for six weeks. It is not known if the injury will require script changes or when shooting will be resumed.
Why the 29-year-old was performing the stunt, Townsend says, "She's just that kind of girl. She's like, 'Yeah, I'll do anything.' But I said, 'The stunt girl is going to start working and not you.'"
Townsend and Theron play fictional lovers in the movie "Head in the Clouds," (search) which opens in some cities Sept. 17.
"Aeon Flux," (search) which was once depicted in a series of animated shorts on MTV, is set centuries in the future. Theron, who won an Oscar for her performance in 2003's "Monster," plays the title character.