Camera - Using the flash to illuminate in camera mode?
The built-in camera app (on vanilla Android and every other device I've personally seen) doesn't support this. The flash will only come on as the picture is being taken, and you cannot set it to provide continuous light when taking a picture.

There are two potential options, depending somewhat on the version of Android you're using. One option would be to use the video mode (with the flash enabled) and capture images as you're recording, then discard the video at the end. You should be able to do this on newer versions of Android by tapping the screen while video is recording. I believe this only works on Jelly Bean (or later), but manufacturers have implemented this themselves in a few cases (ex: The Galaxy S3).

A second option would be to use a third-party camera application. Camera MX, for example, allows you to put the flash into "torch" mode while taking pictures. Simply tap on the lightning bolt icon until the small infinity symbol appears on top of it: