A boom operator is an assistant of the production sound mixer. The principal responsibility of the boom operator is microphone placement, usually using a boom pole (or "fishpole") with a microphone attached to the end (called a boom mic), their aim being to hold the microphone as close to the actors or action as possible without allowing the microphone or boom pole to enter the camera's frame.
Invention of the boom mic
At Paramount, Dorothy Arzner directed Clara Bow's first talkie, The Wild Party (1929). To allow Bow to move freely on the set, Arzner had technicians rig a microphone onto a fishing rod, essentially creating the first boom mic. She did not, however, take out a patent. One year later one was filed for a very similar sound-recording device by Edmund H Hansen, a sound engineer at the Fox Film Corporation.
Often in television studios, the boom operator will use a "fisher boom", which is a more intricate and specialized piece of equipment on which the operator stands, allowing precise control of the microphone at a greater distance from the actors. They will also attach wireless microphones to persons whose voice requires recording. Boom poles are usually manufactured from several lengths of aluminum or carbon fibre tubing, allowing the boom to be extended and collapsed as the situation requires.
Some poles have a microphone cable routed through the inside of the pole, which may be a regular cable protruding at the bottom end, or a coiled cable that can extend with the pole, connecting to a socket at the base into which the operator plugs the microphone cable. The ideal boom pole is lightweight and strong, supporting the weight of the microphone on the end while adding as little weight as possible.
Frequently, a wind-attenuating cover, called a "blimp" or "mic-blimp", is used to enclose the microphone. A blimp covered with sound-absorbing fuzzy fabric is usually nicknamed a windmuff or a "dead cat". In film crew jargon, the gruesome-sounding phrase dead cat on a stick is simply a boom microphone fitted with a fuzzy wind-screen.
The boom operator and production sound mixer may often be combined into a job performed by one person, usually when the crew number is to be kept minimal, such as for documentaries or news collecting, or in low budget productions. The one-man unit is often known simply as a "sound recordist" or "sound man", and would perform all on set sound duties.
The boom operator must decide where to place the microphone based on a combination of factors, including the location and projection of any dialogue, the frame position of the camera, the source of lighting (and hence shadows) and any unwanted noise sources. Often the boom operator will need to be as familiar with the script as are the actors, as they may be required to tilt or move the microphone according to who is speaking.
In productions with a bigger budget, more than one boom operator may be used, with each operator focusing on a different actor.
Having the boom mic or its shadow appear on the screen in a completed picture is considered a sign of poor film-making. Notable examples include the mic's shadow appearing above two crewmen flying a plane in Plan 9 from Outer Space and the mic itself dipping into the frame numerous times in Rudy Ray Moore's film Dolemite. The TV Tropes wiki has a list on its "Visible Boom Mic" trope page demonstrating more examples. Pastiches of bad film-making may also use boom mic visibility to spoof their material.