How they're being underpaid and probably how they think the main star isn't "all that".

Normally extras in the background don't actually say anything when the main actors in the scene are speaking their lines because it would mean that the extras voices would be recorded on film as well - obviously this is not good as you need to hear what your main actors are saying. In a busy nightclub sequence, when they actually film the shot, typically there will be no background sound or music and the main actors will shout their lines to each other (as you would if in a real nightclub) and then when the film is being edited, the background music and noise will be added in to the sound mix and make it seem like it was all done at the same time. Another way of doing it would be to shoot everything with all the sounds and then have the main actors come back months later the re-record their lines in front of the footage to match it up with what they shot at the time - again this is to make sure that what they are saying is heard properly. I think that process is called 'looping' or ADR (Additional Dialogue Recording). But I imagine most of the time they want to make sure they get it done properly on the day and just shoot the lines without any background noise - so the extras will just move their mouths without actually saying anything.

They actually talk about anything they want to.I have worked on set many times and found this was so. There has to be ambient background sounds of people talking and mingling to make the scenes look natural.