I posted another question which sparked a bit of a discussion in the comments. Basically, in a high contrast (high dynamic range) picture, if the difference in distance from the camera to the different parts of the subject is negligible, then does a fill flash actually help bring out details in the darker areas of the subject?
The way I see it, if the difference in distance is negligible, then using a fill flash will add a uniform amount of light across the entire subject. If different parts of the subject are considerably but not extremely different in ambient illumination (say, something like 4-5 EV difference; as an example, in shade versus sunlit), this should help bring out details in the darker areas, since those are illuminated more by the flash relative to the ambient light level than are the highlights. But it's been argued in the comments that that's not the case, since the absolute difference in illumination remains the same with or without a fill flash (which would mean it's all about the exposure settings).
So, which is it? And why?