"ADR - "Additional Dialogue Recording" or Looping as it's also called, is where an voice actor revoices and/or adds to the action taking place on screen.
Accents, dialects, sound alikes, changing one's texture and or vocal qualities are all benefits to increasing your odds to work as a looper.
Remember, the voices they are usually looking for in ADR are real and believable sounding. The more versatile you can be with age ranges, is also an added plus.
Successful loopers know how to project and play to the mic. They know how to watch the action and fill in the dialogue off the top of their heads. They are excellent improvers.
Example of a loop: Brad Pitt is in a diner talking to his girlfriend in the movie. the LOOPER adds to the action around the principle actors, ie the waitress, short order cook, bus boys, customers eating and talking etc....
The looper's job is to sometimes be bland in dialogue so as not to upstage the scene going on. Sometimes you have to scream a blood curdling yell as in a horror movie. Sometimes you have to hoot and holler at a concert or game. You must have your vocal skills ready to work an 8 hour day which could involve yelling allll day long!!!!