So whats a weird mix of the two?
Headers on a set are usually under the guise of a structural member. We often cheat the depth of a concrete beam and can get away with it. They usually wouldn't really be 3' deep, but their mass makes it less obvious.
3' deep headers spanning a set that are only 4" thick are a dead giveaway aswell. A 4" thick wall has limited structural span.
You also wouldn't see a moulded ceiling panel type header design with 24" deep beams intersecting at the center of the room, as this wouldn't be logical structurally.
The trickiest header scenes for me have to be hallways. How do you sell a hallway of headers as real architecture? In a hallway, you see the ceiling more than the walls often. They are often dolly tracked walk & talks. You have to either stick with typical construction standards for the right structural look or do something different.
Whenever I can I put all the money into the ceiling of a hallway set. The new cameras are so much more fun for set designers, as we no longer are dependent on overhead studio lighting. There are huge lighting opportunities in hallway headers. You can get away with minimal wall finishing in a hall that is all about the lighting.
In lieu of lighting the headers, I have found long wide headers sell best.
Its just got to look structurally believable.