Having established an acceptable sound balance, you must ensure the level you record remains consistent for editing purposes. You should ask yourself:
Is the background sound level likely to fluctuate?
If so, how much?
Is that likely to prove obtrusive?
Is there another source of sound likely to cause problems during the recording at this particular location (aircraft, construction, roadworks, etc.)?
You have to anticipate the answers to these questions, but a look around your location before you start recording should help you pin-point any likely sources of unwanted interruptions.
Murphy's Law
Be warned, Murphy's Law operates the moment you start recording (whatever can go wrong, will go wrong), so always be prepared to deal with the unexpected. The minute you run to record, helicopters appear overhead, lawn-mowers and hedge cutters race out of hiding, motorcycles scream round the corner, pneumatic drills immediately thunder away, and you, as the recordist, may be expected to ascertain how long any of these interruptions will last. Whilst tact and discretion frequently save the day, there may well be times when you have to proceed with your recording regardless of the noises off, which could leave you facing some tough decisions with respect to the end product. If the sound becomes unintelligible, or you realize that an edit can't take place within a portion of the recording, you must point this out to production and either suggest a re-take, or some covering sound (see Wildtrack below), or even a complete reposition of the item.
An unexpected interruption can also take the form of a change in background noise. Do beware of computer noise, printers, copiers, fridge compressors, air conditioning, etc. as a background constant. Should this noise suddenly cease, you've got a potential editing problem. The best bet is to turn all these devices off, if possible, before you start recording, then at least they can't cease of their own accord (but for goodness sake remember to turn them on again before you leave, or you'll not be welcomed back to that location). If in doubt, record a thirty second wildtrack before any other activity takes place.
Other noises to beware of are muzak/radios, etc. playing in the background, or any regular noise, like a clock ticking. This type of sound certainly adds to the ambience of the location, but invariably obstructs an edit. So, the only safe way to overcome the problem is to eliminate it altogether. Get the muzak (radio) turned off (see also, Copyright below) and stop the clock. (A word to the wise: by doing this, you may make yourself extremely unpopular. Employees swear they can't work properly without the radio, and the clock is an antique grandfather clock that's been in the family for generations, and has never stopped.
You can guarantee that once stopped, it'll never re-start. Whoever said this job was easy?)
Run to record … and cut!
When you have started recording, do not let any action commence until you have been running (turning over) for at least ten seconds. Similarly, when the action has finished, do not cut immediately, but continue to run for a minimum of ten seconds. Whilst these timings used to be mandatory in order for playback machines to stabilize for an edit, they also serve the (more!) useful purpose of giving a sound lead in and run out, in addition to any wildtrack. If there is an ongoing/passing background noise, such as an aircraft, etc. this should be allowed to die away before cutting. Indicate, by pantomiming ‘hush’, that all on location should keep quiet in these circumstances.
Wildtrack
(Also known as atmos or buzz track, or ‘room tone’ in the USA.)
How to win friends and influence editors! To help overcome those situations outlined above, one minute of wildtrack can save the day (literally). Record the clock ticking, striking the hour, the bells tolling, the muzak playing (see Copyright below), the builders hammering, etc. This background sound can be added at the edit to reinforce the acoustic atmosphere/ambience, or to help ‘sweeten’ an edit (i.e. cover the join).
Whatever your location, I would always recommend recording some wild-track. This is especially important if there's a fluctuating level of background sound, even though it's relevant to the location. During an interview, there may be distant traffic, or aircraft noise, but you can bet your boots when an edit is required later, the noise will be at a peak which needs to be sweetened to become unnoticeable.
Whilst recording, be aware of these inconstant noises off, and if and when they peak during the recording. You must then record a wildtrack with the mics in the same positions once you have finished recording the main soundtrack. Whatever the background noise you heard, ensure that you obtain a complete cycle (cars, lorries, trains, planes, helicopters, etc. passing outside). You'll need to be firm, even 30 seconds seems like half an hour, and you must ensure that the contributors (and production team) make no noise whatsoever during your wildtrack, for it to be of any use.
Copyright
Whilst a soundtrack of any background noise on location is vital, you should be aware that some sounds, music especially, are subject to copyright. You should indicate to your production team on location that they may need to seek clearance for your wildtrack, and also ensure you make a note on the cassette to alert the editor of a possible restriction in its use. If the sound is subsequently included in the edited mix, it's the production's responsibility to obtain the necessary copyright clearance.