James Crocket to CAS forum: 

• Approximately what percentage of the dialog in a movie is ADR? 

• Is it reserved mostly for outdoor scenes or do unpaddable, large, live rooms demand ADR dialog as well?

• Is there a standard or well known system of processing the voices to fit the environment seamlessly?

• Is ADR an absolute last resort that should be avoided at all costs, or just an everyday reality of the job?

• If your sync audio is unusable, it is good to record the dialog wild, true? 

• Is this almost always more successful than looping it in post? (Provided of course that the actor can duplicate his/her lines from the take)

Sometimes a director likes the staging and visual part of a scene, but does not like the way the actor delivered his lines. Again ADR is a useful tool to get a better reading of the dialogue. Then, there is the situation when there is just too much background noise because of location logistics. A really good production sound mixers should generally be able to get al least 90% of the production sound on a feature film clean enough when no ADR is required.  This is what separates the really great mixers from the ones with little experience and no talent. Some directors are very sensitive to the sound portion of a film and demand to use as much location production as possible.  They really would like to have every word original production sound.  Other directors, (especially new to the profession and those getting by with little or no real talent) are not as 
sensitive to the sound.  They have not yet been able to understand the difference between original dialogue and ADR. Is hard to give you an exact figure on percentages, but in general most feature films today will have about 90% original production sound, the rest ADR.  I have worked on features that were 100% production sound (including wild track dialogue recorded on location.  Many of the spaghetti westerns were 100% ADR. 

Carl was right to point out that a significant amount of ADR is done in order to change the actor's performance. Sometimes lines are re-written, or the actor may have had a cold and sounded stuffed-up on the day they shot the scene, etc. The most difficult thing about integrating ADR into most films is that it is unusual for entire scenes to be ADRd.  The poor dialog re-recording mixer is often asked by the Director to stick one or two words of ADR into the middle of a line of dialog which is otherwise "production."  This is probably the single most technically difficult 
process in all of sound mixing. There are lots of techniques used to make ADR sound more like well-recorded production dialog. 

The best way is to record the ADR in an acoustic situation as close as possible to the one on the set, except without the noise of the set.  Using the same mic as the production mic helps. The re-recording mixer uses eq, reverb, digital pitch-changing devices, and lots of other boxes to try to make the ADR sound like production. The dialog and ADR editors have typically also used systems (like VocAlign, for ProTools) which compare an ADR line to a production line and alter the duration of each ADR word in order to bring it closer to perfect lip-sync.  In addition, they've used many traditional techniques, including the one of extending the "room tone" or "outdoor air" under ADR lines so that the production ambience doesn't disappear whenever an ADR line is used.