It is making me think a good idea would be to capture sound with that microphone (compressed) + use a shotgun microphone + boom as well and then the sources could be mixed together... and if a line is missed, it could be covered by the "back up" mic.
It's a shame jrusso has contributed to this thread and influenced your thinking. Now you're going to have to waste time trying it out and satisfying yourself what works and what doesn't. To this end, I strongly recommend you run some real world tests, in the sorts of locations your feature will be set, well before you actually start shooting your film. Wasting time when testing is obviously far preferable to wasting it during actual filming!
I'm not saying that having a "back up" mic is a bad idea, quite the contrary. Even most micro budget feature filmmakers use back up mics (in the form of lavs) and low/med/high budget feature makers use lavs as a backup as standard operating procedure. It's only at the nano/no budget levels that filmmakers tend to use just one mic, purely to reduce cost. Except in certain specific cases (ENG and occasionally as "plant" mics), the use of standard studio/general purpose cardioid condensers is avoided and I've never seen a large diameter studio condenser employed to capture production dialogue. Their sensitivity to movement, noise and other environmental factors normally rules out their use.
Something your tests may highlight is the skill required to operate the boom, one of the most common failings amongst inexperienced or extremely resource limited filmmakers. The widest polar pattern mic ever commonly employed on a boom is a hyper-cardioid, which is relatively forgiving but the tighter pattern mics (used for their improved noise rejection properties) obviously have a narrower "on axis" area, so need to be aimed more carefully. Getting consistent recordings therefore requires at least some skill and in many filming scenarios, considerable skill and experience on the part of the boom op. Providing the actors are seated or don't move around, a fixed position cardioid will make consistency far less of an issue. Compared to a well aimed shotgun/hyper though, it will also be consistently far noisier!
Another point to bare in mind; generally, mixing different mic sources together should be avoided. Although it's fairly common practice in music production, due to how theatrical sound systems, broadcast chains and audio formats work, inaudible phase discrepancies or even desirable phase based "effects" (in music production) can cause havoc in film! Nothing to worry about if you're just making a film for say Youtube distribution but film festival screenings and traditional commercial distribution channels is where you can run into serious problems.
Have any of you guys used the Audio Technica AT897 or the Zoom H4N?
I have used a H4N but not an AT897. The H4N is a handy tool for some purposes but it lacks the features/options commonly employed to streamline the audio post process of long form filmmaking. With additional time/effort this limitation can be worked around but the H4N's biggest drawback is that at moderate/high gain settings the internal mic pre-amps are unacceptably noisy. And, from what I've heard, the AT897 does not have a particularly strong output level, which would necessitate a higher pre-amp gain setting. Of course, what I consider to be "unacceptably" noisy may not be the same as what you consider to be acceptable.
The Postal Service uses a Rode K2 for his vocals and instruments -- I think his sound is not "filmic" though vocally.
You seem to be quite intent on applying music recording equipment and techniques to filmmaking and unfortunately, you've been encouraged in this view. Having started out in the audio industry as a music recording engineer/producer and then transitioning into film sound/audio post, the differences between the two (and the transition time) were far greater than I imagined/anticipated. As I said though, if the target for your feature is say Youtube then you can get away with almost anything, providing your dialogue is at least audible. But, if you have higher aspirations for your feature you're going to need to apply standard production and post prod sound equipment/techniques, which exist because they have proven to be the most reliable/efficient at achieving the required standards.
G
