This is something I was aware of but thought other people might find interesting. From here.

A surprising amount of dialog that you hear in a movie was re-recorded after the film was done shooting. On a romantic comedy, it is around 40% of the dialog. On an action/adventure movie, it is between 60% and 80% that is re-recorded. On a film like Transformers, it is probably between 90% and 95%.

Anyone who has ever done any sort of work with sound capture knows how difficult it is*. Anything can screw up your take. And a microphone sensitive enough to capture a human voice at a distance captures every damn thing else too. Planes flying over. A dog barking a mile away. A train three miles away. Birds. Insects flying by. A car five streets over.

Unfortunately I’m really sensitive to overdubbing and such so particularly when it’s done poorly I can tell and it makes the film difficult to watch. In Ex Machina (my current movie obsession) I only noticed it once which is some sort of record. They did a great job with sound and all else in the film.

There was some movie with Rachel McAdams that I can’t recall the name of now that the re-recording and overdubbing was so poorly done it was nearly impossible to watch.

*I sometimes worked with and for TV news broadcasters in the army.