Dialog isn't just dialog
In our shop, we don't divide the dialog, music, and effects onto different workstations. All the dialog, music, and effects live happily on the same timeline.
The production sound tracks are mostly important because of the dialog in them. But some production sound actually has useful sound effects.
So when there are production sound effects (like footsteps, etc.) which seem usable (and aren't right on top of dialog) we simply move those effects down to a "sound effects" track.
Here you can see how some parts of the production sound recordings are on dialog track and some parts are on "fx" tracks.
Now typically we do not put in "fill" or room tone in-between the dialog tracks. We just add tone on top of everything. There's a number of reasons we do it that way. Firstly, we're putting a bunch of dynamic effects as well as noise reduction on the tracks so the noise level is going to be going up and down all the time anyway.
Also, it's nice to create your own unique room tone for the scene.
You know what? I'll go one further. Dialog is arguably the single most important element in the entire movie. You might be able to argue that visual effects are more important to selling a picture, but the dialog's quality can make or kill a sale.
Ironically, this is true even sales to non-English-speaking territories. Why? Because they watch the movie in English. And many, if not most, of their customers watch the movie in English.
The trick to mixing overall is to get quiet and loud dialog tracks. That's right — loud and quiet. The noise should be quiet and the talking should be loud.
And we also have to do it quickly. So what happens here is that we put a couple limiters strapped across the dialog buss — the channel all the dialog runs through. Then we put some gentle multi-band expansion over the dialog buss. And THEN we throw the whole thing through some single-ended noise reduction. And maybe even more single-ended noise reduction.
Is this the perfect way to do it? Not at all. What you should do is manually adjust every segment of dialog so that it's loud and clean. But instead we send all the dialog through a chain of processing which takes care of 90% of the issues that come up.
Some pieces of dialog are noisier than others — those sections will get more single-ended noise reduction applied to them.
And furthermore at least one of the sound effects tracks will be dedicated to "production effects" — that is, the sound effects recorded on set. Those sounds will also have a single-ended noise reduction plugin applied to the entire track. That way the nicely quieted dialog tracks will match the now nicely quieted production effects.