most don't know (because of how well its done these days) but all audio in film is a re-record (its called ADR, automated dialogue replacement). Not a single line of dialogue you are hearing is coming from their mouths. Its recorded on set as a guide-track and the actors dub over it in a studio at a later stage. The sound mixer gets it to sound natural using convulsion reverb, so the sound sounds like its taking place in a room similar to what you see.

So lets say the scene is in a bar. They record the sound in the studio, and then give it a bit of reverb which emulates a room like that on the movie. You see every thing we see, we sort of have a guess how it should sound in that space. If what we see doesn't match what we hear, it seems fake.

This movie has very little reverb added. Its so dry. It sticks out as a studio recording. And the lip sync doesn't work that well at times.

The lip sync would be harder to fix (CGI, editing the speed of the audio) but the reverb is an easy fix. I think they should do a digital remastering of this movie for subsequent releases and make the dialogue sound like its taking place in the venues we see.