By "overexposure" I'm only talking about maybe a stop and a half higher than you normally would set it. Something some people might consider normal exposure. Not anything extreme like adding dB or upping five stops. I've done this before, and it looked great. This time I actually know what I'm talking about.
Commonly, if your footage looks just a tad blown out, the detail is still there, it's just not obvious to the human eye. Usually, lowering the brightness a bit will help the fine details to present themselves. Considering your white balance is set correctly, you should retain true color, even after brightening the shot.
From my experience with the GL2's output footage, your end result will have less grain/abberation from tweaking in post if you choose to slightly overexpose and, in post, add a -.05 darken rather than slightly underexpose, adding an, oh, let's say +.07 brighten. In this case, the brighten will bring out the grain in the image whereas the darken actually brings the black point of the footage to a truer tone.
And the comment on white balance wasn't something I was advising doing for your actual shots, it just helps you learn how the white balance settings work which will help you down the road when you're stuck trying to set true white balance, which is what you should always do.
As for Hybrid's advice about going auto just for this one shot: I wouldn't. It's almost ALWAYS obvious when autoexposure is used, and it's a hallmark of amateur cinematography.
Tarn wrote:If you shoot carefully and plan shots, then D-f-N can be very successful. Even without that it can still work well - it's mostly a subliminal thing anyway. You don't need much for an audience to buy it.
Forgive me for disagreeing here, but I've yet to see believable day-for-night from any amateur miniDV shooter. A prime example is Get Lost. While I loved most of the movie (and the high contrast in the flashbacks!) I had no idea that the darker scenes were attempts at day-for-night. I'm not trying to say they were poorly done by any means, rather, there simply wasn't enough time taken to establish the shots as nighttime and to clean up the grading and filters a bit more.
D-f-N is a nice idea
, but I can almost guarantee you it'll never look how you want it to. Properly lighting a room is a much more substantial idea, and it'll probably look better in the long run. Plus, with D-f-N, you're sort of stuck doing a 'guess and check' with framing of bright things such as windows and the sky, as well as the extensive, grueling post-work. Lighting beforehand might be better.