It is preferable to use the production sound (the sound recorded on the set). When the set is noisy due to generator noise or other reasons and the production sound is unusable it is becomes necessary to re-record the dialog.
The process of re-recording the dialog after the shoot is completed was called "Looping" (some still call it that). Before the digital age they would create a film loop of the piece of dialog to be re-recorded and play it over and over until the talent matched the original dialog performance exactly.
The process is now called Automated or Automatic Dialog Replacement (ADR). The process is somewhat easier since the ADR editor can use softwares/plug-ins such as Vocalign to conform an ADR performance that is not "perfect".
In my experience each line is preceded by two beeps; a "2-pop". Otherwise there is nothing to assist the talent to stay in sync with the dialog, they must do it "the hard way", mimicking their own (and in some cases another actors) performance.
Directors usually prefer the production dialog. Most Hollywood films are about 25% ADR; action films tend to have quite a lot more due to all of the noisy effects, etc. used on the set. I've heard that the LOTR films were about 90% ADR. It is an unusual and very talented actor whose ADR performance has the passion of the original. |