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.
Through at least the 1980s it used to be very normal in some countries (esp Italy) to shoot the entire movie without sound and add in all the soundtrack later. This is also why non-Italian actors appeared in some famous Italian movies. Two of the three prinicpals in Fellini’s La Strada were non-Italian and I’ve read that even his wife Giulietta Masini’s voice was dubbed by someone else in the final print.
Also, any time you see a scene in a disco or place where people are dancing the scene is shot without music (I was once an extra in a scene set in a bar with people dancing until I got bored and left).