I'm not a sound mixer, but I've done a lot of it, which is somewhat of a normal aspect of operating camera on low budget projects. As such, I've learned a few things about recording clean, usable sound on location.
Micing: The pro approach
Normally, location sound mixers like to employ a wireless lav mic and a shotgun mic simultaneously for each actor. Reasons for this have to do with safety and have to do with conditions on location.
Sometimes a wireless lav mic will be subjected to occasional radio interference which can create short bursts of static over dialog. Having a recorded channel from the shotgun mic can alleviate having to shoot another take if you have a clean recording with it.
Also, a lav mic's battery might start going dead during a take and having the redundancy of the shotgun mic can save you, especially if the take is a keeper. The last thing you want to do after a perfect take is shoot it again, even though you will anyway ;-)
Having two usable audio channels to employ in post each with their own different sets of characteristics is favorable. As an editor, I have repeatedly switched from one channel to the other as needed.
Using a single shotgun mic
I've shot multiple feature length films with only a single shotgun mic and this can work fine if you're careful and always record "wild lines" whenever needed to use in the edit. For example, when shooting a very wide shot where you can't get close enough with a shotgun mic, or when the actors action makes it too difficult to record and their face is not visible.
Issues with only using a shotgun mic are room acoustics (echo/reverb), picking up ambient noise, and too much distance from actor depending on the shot.
Using lavalier mics
The key thing with placement of a lav mic is to eliminate ALL clothing movement near the mic by taping the clothing to the actors skin, and taping all layers of clothing together where the mic is places. This can be done with tape loops made from paper tape (black usually, unless it's visible through light colored cloth) or moleskin (available at drug stores).
As for where to place a lav mic, it's not always the same on every actor as it really depends on what they're wearing more than anything. Sometimes it can be placed under a collar just out of sight of the camera. Sometimes it can be taped under the garment (facing forward and as close to the mouth as you can get it).
Try not to place a lav mic under too many or too thick layers of cloth or the sound will become muffled.
Generally, placement of a lav mic is most problematic with women because they tend to wear low cut tops which sometimes leaves almost no place to hide a mic. One trick in this instance is to cut a makeup sponge wedge part way (to run cable through), place the mic so it's just sticking out the top (larger end of the wedge), and place sponge/mic in the actresses breast cleavage.
The cable can then be run around to her back and out of sight.
ADR ("Automatic Dialog Replacement") is generally a regular practice with studio level motion pictures. Certainly location dialog is ALWAYS recorded, but large budget films go for spectacular audio quality, so they will re-record the dialog with the actor in a recording studio as they watch the looped scene on a large screen so as to match their new dialog sound take with their mouth in the scenes.
Scenes or parts of scenes are looped for the actor so they can repeat their lines until getting it perfect.
This process is actually quite simple to achieve even on a low budget, but of course it's always desirable to get clean location dialog recordings in almost all cases. Sometimes it's just not possible, as in the case of shooting a scene in a noisy environment (office with loud air circulation, bar/restaurant with loud coolers, exterior location with loud traffic, etc.)
This is generally forgotten about, but oh so important during the post process, especially if ADR is required. Generally it's adequate to record 30 seconds of room tone while still on your set with everyone still in place and not making a sound.
Having this ambient sound recording makes it possible to create acoustically matching dialog by mixing the room tone with your ADR recording. Also, it can come in handy for filling in gaps in an audio channel if a noise is removed (because you lose the ambient tone by doing so).
Of course, sometimes the location is noisy and if so and you intend to loop dialog later, you'll have to get quiet room tone from somewhere else (that matches the location to some reasonable degree.)
A word on noisy locations
With low budget films, you generally are at the mercy of whomever's property you're working on (or neighbors). That said, it generally means, NO you cannot shut off the walk in freezer, or NO you cannot make those people be quiet (because they're open for business), or NO the lawn crew will not stop cutting grass, etc. In cases where you either secretly or openly shut off machinery that hums in order to record clean location dialog, MAKE SURE you turn the machines back on.
Write a note to yourself, tell at least one other person, AND somehow make it impossible to leave without turning the machine(s) back on. I worked on a feature film once in a restaurant where certain coolers were never turned back on after shooting scenes, which resulted in $1,400. damage (that production had to reimburse for).
The most important thing
No matter what, keep in mind with any mic use, you want to place the mic as close as possible to the actors mouth while making sure they are not breathing on it. Take notice of where they will turn their head predominately during a take and place the mic(s) accordingly.
The closer the mic is to the actors mouth, the quieter the background noise (ambience) will be, which is exactly what you want.
©2011 Chris Santucci