Ambient sound | Producing a seamless soundtrack
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3. Shoot several takes of every setup
You should definitely do several takes of each scene, regardless of how well the actors are performing, because in this way the entire scene will be covered with good sound, even if no single take was flawless from beginning to end (they rarely are). If there are cars driving past outside, this will definitely be the case. With multiple takes, finding a clean version of any one section will be a breeze. You can (and should) listen to the sound during and after each take on location, but I guarantee there will be little annoying sounds that you missed in the hustle and bustle of shooting. That's when you'll be glad you did several takes.
I once shot in an interior location where the ancient heating system would occasionally emit a diminutive but very annoying “ding”. Not only was this not noticeable while shooting – it also occasionally overlapped with words, which was subtle but annoying. I was able to produce a truly flawless soundtrack in post-production by replacing the word or words that overlapped with the noise with clean versions from other takes - all done manually with Adobe Premiere. You'd be amazed at how seamless you can make your soundtrack by using such techniques. Time-consuming maybe, but immensely worthwhile.
If the extraneous noise, like a car passing by, happens in between lines of dialogue, it's much easier: just splice out the noise and replace it with clean ambient sound. Which brings me on to the next sound recording tip:
4. Record at least 30 seconds of ambient sound
For every location and every shot in that location, be sure to record at least 30 seconds of ambient sound. That means shutting everyone up and recording 30 seconds of silence. Of course it's not real silence; it is ambient sound, and the “silence” will be different for every location and every setup in that location (because the loudness of ambient sound depends in part on how close the mic is the actors, which in turn depends on how the shot is framed).
5. Disguise the sound cuts by not aligning them with the video cuts and editing creatively
For a truly seamless soundtrack, you will also have to disguise the sound cuts by not aligning them with the video cuts - in other words, the video and audio cuts should be staggered. The reason for which this is necessary is that the quality of the ambient sound may vary between setups and throughout the shooting day, and this change in background sounds will be audible when you cut from one shot to another. You can disguise the cuts by making them coincide with a louder sound (such as someone beginning to talk). I once shot in a location where it was quiet in the morning but full of loud birds in the afternoon. Making those shots match required a lot of creative editing, but it worked.