I had an inquiry into the use of lapel mics in a crowded scene and whether or not sound would bleed into the mics causing distortion and cancellation. I remember watching the great Altman's Nashville and how he used lapels constantly and yet they sound fabulous and this is before all the whistles and bells we have today for tidying up a scene. My question is, in a crowded scene of principle actors where you have an over head boom for general dialogue and lapels for actors a little bit off the axis of the boom and all the actors are speaking rapidly, is there danger of this "bleeding" occurring? Hope this is clear.
I would be sure to mix the radio mics, learn the script beforehand or have one next to you so you can crossfade seemlessly between them as actors pick up their lines. Also, if there are a bunch of background actors doing walla, see if you can have them not do that. This is standard practice, group walla is always looped/cut later.
There is not necessarily any distortion or cancellation, but with multiple microphones you always get a little bit of phasing. The way to get out of this is by actively mixing the microphones, so the characters, who don't have lines, are lowered in the mix. It is nut just problems like phasing, you get rid of that way, it is also the most common problem with lapel mics: cloth rustle!
There is not that much "whistles and bells" to mixing. If the scene is following the script, you will be able to mix the scene for the editing while recording. If it is more improvised, you will have to accept more noise, as you can not mix as aggressively.
Always record the raw channels as well, so you can do the dialogue editing properly afterwards. When editing dialogue I normally stick to the boom, and use the lapel mics only when necessary.
I would boom it with something less directional like the MKH-50 but it really depends on the script and what you want to achieve. You will probably get some phase cancellation as Morten has pointed out, but there is the 'invert phase' function on mixers like the Sound Devices 302 or 442, so that could help some. The bleeding is really dependent on the gain settings and the proximity of the actors to one another. Think of it as when recording drums, everything bleeds into every mic, so the only way you can get some isolation it to either add a noise gate or record them separately. Don't know if a dynamic mic is usable for your situation, but it's something to consider. Perhaps all the actors could carry a slurpee for you to hide the mic in. :-)
With distortion, I reckon you might break a couple of finger nails if one or two of the characters started shouting and screaming though. That would bleed into the other mics, and if you're mixing it down, then you will run a high risk distortion. However if you are recording discretely to different tracks, then you won't have to worry.