How much of a movie's sound effects is a foley artist responsible for? Are all non-dialogue sounds rendered by them? How is this decided?
Foley Artists are responsible for a lot of different types of sounds; mostly natural sounds like footsteps and the sound that certain clothes make while moving in them, but pretty much anything that you can fit into a foley studio. In addition to Foley Artists there are also Sound FX Designers whose job is to create the sounds that can't be naturally or easily recorded.
The following text (which I grabbed from marblehead.net) does a pretty good job of describing who's responsible for what:
Foley does not cover sounds like car engines, explosions or other mechanical stuff - driving a car around in the studio or blowing up a building is usually not possible although we have tried! We don't do birds, laser blasts, dog barks or rain storms either!
While a Sound Editor can do very precise and repeatable effects, they have a harder time when it comes to footsteps for example, since every step is different and unique, the pace changes and the mood of the step is always different. With a good pair of shoes and years of practice, a Foley Artist can perform an actors walk perfectly on the first take while making it sound natural!
When it comes to how much of a film's sound effects that are actually added during post-production, an excerpt from an article featuring an interview with Oscar-winning sound editor Richard King tells that it's almost all of them. Though, it most likely differs from scene to scene, and movie to movie.
"The audience should believe that everything they hear was recorded the day the movie was shot," says King.
That's far from what actually happens in a feature film. The majority of the sounds that make up a scene has to be recorded and inserted after production by a team of sound designers. The art of recreating sound effects on a stage for a film is known as foley art.