The ideal place for the microphone is three to six feet from an actor. Not only is the actor's voice stronger the closer you place the microphone, but it also allows the recordist to lower the incoming volume, thus lowering any system noise. The danger of putting the microphone too close to the actor is the possibility of creating a hiss on any spoken “s” sounds or popping caused by a “p” or “d.” This can be minimized with the use of a foam cover over the microphone.
Things That Go Boom!
Separate from any pyrotechnics work, a boom is a long pole, mounted or held so that the microphone is just out of camera range. It's important for the boom operator to work closely with the camera operator and the best boy to make sure the microphone is not seen and doesn't cast a shadow. Shadows can be eliminated by moving the boom to a new position, such as below the subject, pointing upward.
Lavalier and Radio Microphones
Sometimes it's simply impossible to have a microphone close enough to hear an actor without it being in the picture. For these shots you might want to consider a lavalier microphone. The lavalier is about the size of your fingertip. It can be clipped to an actor's clothes, or even taped to their skin to avoid the noisy rustle of fabric. The difficulty lies in the long thin cord that connects the microphone to the recorder. In some cases, this can be threaded through the subject's clothes, out of sight until it reaches the floor. But keep in mind that this limits the actor's movements and the director's choice of camera angles to avoid revealing the cord.
A better solution is the use of small radio microphones. These have a small transmitter that can fit in the subject's pocket or clip to the back of a skirt. Each microphone has its own frequency and its own feed into a mixing unit, which is attached to the recorder.