The Little Mic That Can
Ever wonder how they got such great sound without a boom? Whenever it's impossible, or even impractical, sound mixers turn to Lavalier Microphones (you may hear them referred to as "Lav Mics.") They are tiny, easy to hide, and are used on most film sets.
Lav mics, when properly placed, sound fine. In the right hands a boom gets great sound, but there are times and places where having a boom is not practical. Extremely wide shots for example, or conversely, extremely cramped spaces.
Because the mics are so small, the sound can appear a bit compressed, and if NOT properly placed they will pick up the movement of clothing, and body hits (thumps) that you may not want. Experienced production sound mixers generally know how and where to place the mic to minimize the rustling from the clothing. It’s also something that the costumer takes into account whenever possible. They can work with the sound mixer to find the right material and the best placement of the mic.
On most films, they will try to use both lavs and a boom .
When all else fails, and it does on occasion, then the dialogue will be “fixed” in post. First with filters and a proper mix, and if that doesn’t do it then with ADR. ADR tends to be the last attempt to make the dialogue work because it rarely matches production, and you tend to spend a great deal of time and money recorder something pristine, only to muddy it up so that it will fit in with the production track.