The sound department plays a tremendous role when considering that there are really only two major elements that comprise a movie; Picture and Sound. The sound department is responsible for capturing the best possible audio and dialogue throughout the actual production (not post-production). The boom operator assists the sound mixer while also positioning and holding the boom pole during the filming process.
This person is responsible for properly positioning the microphone boom pole during the actual filming. The boom operator is the assistant to the sound mixer. Many times the boom operator is required to hold the boom pole for several minutes at a time, which can be physically demanding. The boom operator must also be able to follow the actors movements while staying clear of the camera and lights. This makes it a challenging job for achieving the best possible audio.
The sound mixer is head of the sound department and is responsible for leveling, monitoring and recording of audio during production. The sound mixer / recordist decides which microphones to use as well as placements of the microphones. This person can also mix the various sound tracks and audio signals in real time. A film sound mixer supervises the boom operator and/or sound utility person.