In traditional narrative film production, the sound crew consists of at least two members - the mixer and the boom operator. There are numerous other terms for the mixer: recordist, engineer, operator - or, in France, chef du son.
Beyond that, the crew is often joined by a 'swing' member, or cable wrangler.
The mixer is in charge of the crew (the 'key'), and is responsible for the recording of the audio signal. The boom operator is responsible for the placement of microphones (including wireless), and works closely with the mixer to make sure they are placing them optimally.
Questions for the Sound Mixer
-is the signal being recorded?
Most professional recording devices allow for 'confidence monitoring', which means what you are hearing in the headphones is actually the post-recorded signal, ensuring that the audio is being captured.
-have all ‘noises’ been eliminated or minimized?
Because audio needs to be edited, it is critical that there be no unwanted background sound - it will clash in the edit and make the edit awkward and distracting. So the recordist needs to do everything possible to isolate the sound he or she wants from all the other sounds. Mixers go to superhuman lengths for this: shutting down traffic, turning off all ventilation and nearby appliances, covering noisy film cameras (and their operators) under heavy blankets, and baffling hard surfaces to reduce echo and room reverb. Think of exactly what it is you want to record - and then do everything humanly possibly to get rid of everything else. ***Note*** - no matter how 'realistic' a soundtrack in a film may seem, it is important to understand that everything about it has been synthesized from separate clean recordings. It is impossible to mix sound across an edit without giving away the change in ambient tone, so if you don't hear edits, then the sound has been faked. (Though it may be faked with 'real' location sound, laid in to cover the edits. See the next point...)
-has lots of room tone, ambience, and wild sound been recorded?
Because of how difficult editing sound can be, it is critical that you record lots of that background sound, to help smooth over the edits in post-production. Even if it sounds quiet to your ears, that's still a unique 'tone,' and you'll never be able to replicate it exactly. Remember that room tone and ambience change often - time of day, weather conditions, traffic: all these things affect tone. And everything contributes: lights make noise, the number of people in a room (even if they're being silent), the size of the room, or the proximity of trees or water. You can't record too much tone. But you can record too little.
-is the signal ‘clean?’
Not only do unwanted noises and background sounds need to be listened for, but other obstacles to 'clean' audio: hiss, hum, radio spikes, mic bumps, and 'operator noises' (boom operators need to stay well-fed, otherwise their stomachs will rumble at the worst time...) Quiet the set, and listen very carefully to make sure everything is coming through cleanly.
-is the level bouncing healthily?
Remember, there are lots of ways to measure the energy level of an audio signal, and from device to device, and from mixer to mixer, you will find different requirements. But at the most basic level, be sure that your levels have bounce - because at least you know then that you will be in a healthy range. Don't get obsessed with landing every voice peak on -3 dB, or worrying if it dips into the red, because meters change, numbers are inconsistent from device to device, and at this stage paying attention to that sort of thing distracts you from listening to the quality of your recording. Just make sure that the needles aren't pinned either too high or too low on the meter - they should bounce. And then listen.
-are there any variations in the background noise?
Did the refrigerator just turn back on? Is the camera, on the closeup, more audible now that it's nearer the actors? Conditions change, stay on top of it. If things change, you need more tone, so the editor can mix the change in.
-is the microphone being placed properly/ is there ‘presence?’
The boom operator needs your guidance. It's your job to make sure they know where they should be. Are they missing a line? Are they handling the boom too roughly? Are they on the vocal axis of the subject, and are they using the axis of the microphone properly?
-is there adequate power for the equipment?
Don't start a take if your battery is about to run out. Don't go out on a shoot without sufficient power, and know how to find more if you run out. What kind of phantom source does your mic require?
-(documentary/improvisational) is the camera missing something important?
On many kinds of productions, the sound recordist is an important set of eyes for the camera operator/director. For one thing, you hear what everyone is saying, and a director or camera operator should rely on the sound recordist as a source of information about where to look. Other times, the camera operator can be led by the boom operator, by listening to the camera feed and hearing who the sound crew is picking up. In the occasional scenario, the crew may also rely on the sound crew for security, as they can keep an eye, and ear, out for the unexpected.
-have the lines been read properly?
Finally, the sound mixer has to be tuned in to whether a script is being followed. As many people as there are to keep track of this sort of thing, sometimes the mixer hears it best, and will be needed to check the performance. Beside getting the lines right, the sound mixer should also be listening for other elements of a performance: is the actor thirsty, because their lines are sounding very sticky; is the actor hoarse; is the actor reading at the right level for the scene? (eg, in a night club scene, where loud music will be added in post, the actors may speak quietly on set because the music isn't there yet - but the mixer needs to be aware of the final intention of the scene, and make sure the actors raise their voice so it sounds correct when the music is added). Anything that affects the audio experience of the edit is the mixer's responsibility.
The mixer is effectively working for the editor, by providing the editor with the raw materials of a good sound edit.
Questions for the Boom Operator
-is the microphone as close to the subject as allowable?
-is the microphone’s axis intersecting with the subject’s axis?
-is there ‘presence?’
-is there undesirable background noise/ is the signal ‘clean’?
-what are the subjects going to do next? How to react?
-what is the camera going to do next? How to react?
-am I in the most comfortable position to maintain for a long duration?
-have the lines been read/ action performed properly?