If you were to watch an isolated clip of Bonnie and Clyde—say, one of the car chases set to twang-tastic bluegrass music, you'd be forgiven for thinking that this film was a comedy. There's nothing more hilarious than seeing a cop car roll down a hill to the strumming of a banjo. (No, no one gets hurt in that scene. We're not monsters.)
This music was principally the work of two bluegrass and country musicians, guitarist Lester Flatt, and banjo player, Earl Scruggs, who were also known as the Foggy Mountain Boys. And the piece used most often in the film is perhaps their best-known work, "The Foggy Mountain Breakdown," written and first performed in the late 1940's.
By the way, Flatt and Scruggs were major figures in bluegrass and country music for decades—from the 1940's to the 1970's. In addition to "The Foggy Mountain Breakdown," another extremely well-known piece they wrote is "The Ballad of Jed Clampett," the theme song for The Beverly Hillbillies television show in the 1960's.
People talk about this musical choice so much because, in many cases, people didn't know what to make of it being used in this film. They'd hear this music at—what many considered to be—the oddest times. A bank has just been robbed. Gunshots have been fired. People have also been wounded or killed. The Barrow Gang's getaway car rolls out of town. People in the cars keep shooting at each other.
It all sounds dead serious, doesn't it? But then we hear this music. It's a fast-paced tune that's fast, light, fun, and almost comical. It seems surreal—and distinctly out of place.
Well, all of that was intentional. Drawing on some of the French New Wave characteristics that had impressed him, director Arthur Penn wanted to make a point. For Bonnie, Clyde, and the other gang members, all this—the robberies, car chases, etc.—is a big game at first.
They're like children playing cops and robbers with toy guns. Bonnie, Clyde, and the others are being portrayed as little more than children in a fantasy world. They don't see that what they're doing will have serious and perhaps deadly consequences. For the moment, they're just living in the moment, having tons of fun and enjoying their thrills and spills.
As the story continues, however, the horrible violence becomes more real to the members of the Barrow Gang, and the lighthearted music isn't used any more.
Finally, the use of this music in the car chases was hugely influential. Many movies and TV shows copied it after Bonnie and Clyde. But, nearly all just used it for comic effect, without the ironic undertones Arthur Penn had in mind when he inserted it in Bonnie and Clyde.