Without the Foley artist, movies would be, well, boring. And very quiet. Field recording during filming is good for picking up dialogue, but even the most expensive microphones are not capable of recording sound in an uncontrolled environment in the same way as the human ear hears them thanks to that incredibly powerful Signal Processor called the human brain. Add to that the expectations we movie fans have about how our favorite films sound and you get the need for the unheralded Foley artist.
Sound field design ranges from knocking two coconuts together to simulate the sound of Moe smacking Curly and Larry's heads together, to filling a soccer stadium with people and asking them to chant, as Peter Jackson did for battle scenes for The Lord of the Rings.
Foley, the specific art and science of sound field design for movies, is named after Jack Foley, who offered his radio experience to Universal Studios in 1927. Warner Brothers had recently released the first film to include sound (the talkie) and Universal knew if it was going to compete it was going to have to release a film with sound as soon as possible. At the time, Universal was in production on the silent film "Showboat" which mid-production was turned into a talkie. Foley's crew projected the film on a screen and added the sound effects in real time, making sure each door slam or footstep was in perfect synch with the on-screen action. Foley continued as a pioneer in the field of motion picture sound field design until his death in 1967.
Watch this clip of the Three Stooges:
Notice how the Foley artists only used one sound to mimic the mud slap? As you watch the video, take a mental note that regardless of distance or energy (or what surface the mud hits), each and every mud slap sounds exactly the same.
Now watch the clip again.
This time, you knew the slaps were fake, and your perspective of what you were watching completely changed. You cannot unhear the fact that the sound is not connected to the on-screen action.
The fun thing about sound effects is that most of the time your brain will not question what it's hearing in relationship to what it's seeing. Then, once the unnatural connection is pointed out, you cannot "undo" the information.
Thankfully, sound engineering has come a long way since the early days of the talkie, but the principle is still the same: trick the viewer into thinking what they are hearing came from what they are seeing, and not a soundstage on a movie lot.
So, the next time you watch a movie that sounds great, thank your friendly neighborhood Foley artist.