Sometimes the audio on set can not be used and the actors dialog need to be replaced. Perfect example of this: How come you can hear the actor's dialog so clear as they are standing next to a helicopter running, ADR replacement.
An actor watches the image repeatedly while listening to the original production track on headphones as a guide. The actor then re-performs each line to match the wording and lip movements. Actors vary in their ability to achieve sync and to recapture the emotional tone of their performance.
James G. Stewart, head of post-production at RKO Studios, talks about one such challenge when working with Orson Welles on "The Magnificent Ambersons."
There were six principals involved in the dialogue, and I recorded each one separately to the picture. This was done without Orson being on the stage. I then combined these tracks and rerecorded them with the necessary motor noise of the old-type automobile.
On running the result with Orson, he said "It's all right technically, but it's no good from the standpoint of realism. I don't feel that the people are in the automobile. There's no sense of movement in their voices; they're not responding to the movements of the car. The voices are much too static."
So I went back to the recording stage and redid all of the lines. This time they were done with the actor or actress and myself seated on a twelve inch plank suspended between saw-horses. As we watched the picture I simulated the movement of the car by bouncing the performer and myself up and down on the plank. After a week of bumping, I had a track which I then rerecorded and ran for Welles. His only comment was "That's very good". Orson was not given to exaggerated praise of anyone's efforts.
Binary Recording Studio a Video Production Company in Bellingham Wa.