In conventional film production, a production sound mixer records dialogue during photography, but several uncontrollable issues, including traffic and animal noise, during principal photography can cause the production sound to be unusable.
When the film is in post-production, a Supervising Sound Editor or ADR Supervisor reviews all of the dialogue in the film and rules which actor lines will have to be replaced using the ADR technique.
ADR is recorded during an ADR session. An actor, usually (but not always) the original actor on set, is called to a sound studio equipped with video playback equipment and sound playback and recording equipment. The actor wears headphones and is shown the film of the line that must be replaced, and often he will be played the production sound recording. The film is then projected several times, and the actor attempts to re-perform the line while watching the image on the screen, while an ADR Recordist records the performances. Several takes are made, and based on the quality of the performance and sync, one is selected and edited by and ADR Editor for use in the film.
There are variations of the ADR process. ADR does not have to be recorded in a studio, but can be recorded on location, with mobile equipment; this process was pioneered by Matthew Wood of Skywalker Sound for The Phantom Menace. ADR can also be recorded without showing the actor the image he must match, but only by having him listen to the performance. This process was used for years at Universal Studios.