One quick addendum (came to me on the commute to work this morning). Jay mentioned this, and I forgot to.
It's not the case that only your focal area has the most interesting color, contrast, edges and detail, with all of that muted everywhere else. It should have some of that, different from the treatment elsewhere in the picture.
So it may well be that you'll have an entire piece that is brilliant in hue, or perhaps flat in value. In the latter case, say, you simply may not be able to "punch up" the focal area with a lot of value contrast (unless you invent it). That's fine -- just punch it up with more color than the other parts of the painting, or perhaps more detail.
In some way, distinguish the focal area from the rest of the picture, to catch the viewer's eye and say, "Here -- this is what I most wanted to show you."
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