If you plan on doing the noise reduction yourself I would again recommend the book Dialogue Editing for Motion Pictures: A Guide To The Invisible Art by John Purcell. The latter half of this book goes into great detail about the art of noise reduction and dialogue mixing.
All dialogue tracks are going to have undesirable noise to some degree. Even projects shot a on soundstage can come back with various blemishes that will need to be removed before the mix. The process of reducing or eliminating the noise in a track is a delicate dance between what you have to do (because that is the take that you need to have in there), what you can do given the tools available and the type of noise being addressed, and what you can get away with as far as noise being left in, or artifacts of noise reduction.
Its very important to listen on a high resolution system when doing noise reduction. Small computer speakers or noisy rooms make for terrible noise reduction decisions that you will later regret, so make the effort to find a respectable playback system and a very quiet room when making noise reduction decisions. Also remember to listen loud enough to hear the artifacts you may or may not be creating.
We discussed in part 3 what kinds of noise you will and will not be able to get rid of during this part of the process. Of all of the tools that you have available for the purposes of de-noising production audio, your EQ is going to be the most useful. EQs can lop off all of the low end in a recording that doesn't actually represent anyone's voice, and can be used to surgically get rid of certain static sounds like electrical buzzes and light hums.
The second best tool is a multiband expander like the Waves C4. Multiband expanders break the audio into multiple frequency spectrums and then can turn down the parts that aren't actively part of the dialogue being spoken by a few decibels. They aren't panaceas, but they are often part of a good dialogue cleanup processing chain.
Noise Reduction Software
The last and most dangerous is tool is the actual noise reduction software. Noise reduction software works by sampling the offending noise and then using that sample to remove all of the audio that resembles the noise footprint. Noise reduction software can be very useful when used in moderation, but it won't solve big sound problems and can often leave digital sounding artifacts behind as the noise in the recording shifts and moves around.
When using or evaluating noise reduction software keep in mind that your audiences will be far more forgiving of some extra noise being left in than of some unnatural sounding digital artifacts being introduced. The reason is because the noise that the world makes around us generally sounds natural and can be easily tuned out by the brain and forgiven. By contrast, noise reduction software artifacts are very unnatural sounding and can draw attention to themselves even in small doses.
The general rule of thumb with regards to noise reduction is that less is more. If you find yourself or your editor doing wild and weird sounding EQs, if you hear a gate or expander chopping in and out, or if you hear digital noise reduction artifacts, then you should take that as a sign to back off on the processing a little.
Even if you aren't doing the dialogue cleanup yourself, its important to be able to make good decisions as to what is and is not acceptable in your film. Too much noise left in and you risk your audience missing or mishearing a critical piece of dialogue. Too much reduction and you risk drawing attention to the dialogue edit and breaking the illusion.