Apart from the three lead actors, all the other extras and bit parts were street cast on the day.
The final beach scene was filmed on a small island on the East River (right off 22nd St), which only appears when the tide is out. A homeless man tipped the filmmakers off to the location.
The 70s-looking orange tint present throughout the film actually happened by accident, when a camera equipment room member convinced the filmmakers that it would be needed to make outdoor lighting (sunlight) look "normal" on film.
The detail of most scenes was improvised on the day, with actors encouraged to come up with actions for their characters to do.