The whole film was ad libbed, in a way. Hal Needham, Burt's good friend and famous stuntman, came up with the concept. Jerry Reed was originally offered the starring role, but actually wanted Burt.
He knew it would be a bigger hit with Reynolds. Reed wrote the theme in 15 minutes, and it was by far his best selling single. When Needham first heard it, he told him not to change a note.
Burt picked Sally Field, who was surprised, because she didn't think she was attractive enough. Most of the script wasn't even finished when Needham started filming. By the end, there was only one Trans Am left intact, and they had to push it around the set!
It actually came out the same week as Star Wars, and was a close second to it in theaters. They both played for almost a year, often in a double feature.