Wes Craven just announced in his twitter that Scream 4 is facing one of the hardest processes in filmmaking – the ADR or Looping.

It is the re-recording of dialogues that "cannot be salvaged from production tracks", according to this.

"Looping originally involved recording an actor who spoke lines in 
sync to "loops" of the image which were played over and over along 
with matching lengths of recording tape. ADR, though faster, is still 
painstaking work". 

"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".