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Take some 3D objects and using your on board flash as fill start with the OCF directly over the camera at a downward angle of about 45 degrees (matching natural modeling). Then move it in 15 degree increments in a 360 degrees around the objects. Compare the shots and observe which angles create the more realistic illusion of 3D shape and space in the 2D photo. Next try the same experiment with the OCF at 15 degrees vertical and then 0 degrees.
If you can find a person to assist have a figurine with a human face try the same exercise. Try 45, 15 and 60 degrees vertically and note how the brow shades the eyes (or not).
What you'll find is that for most objects, including heads, the illusion of 3D is strongest when the OCF flash is behind and to the side creating "crescent moon" highlighting on the objects. The objects will look more "normal" with the flash at 45V vs. 15V or 0V because natural light , our perceptual baseline for "normal" isn't typically that low in the sky.
With a person you'll find it that while the OCF in back creates a nice 3D illusion of overall shape of a head just the flat light from the fill in front doesn't look normal. That's why the convention for portrait lighting is to place the key light in front of the face about 45 degrees vertically to get past the brow and into the eyes and either centered or to the side to control where the nose and other shadows fall creating 3D shape clues on the front of the face. When a third flash is available adding from behind as "hairlight" adds the same overall impression of 3D shape as seen in the still life objects.
Once you decide on the strategies which create the the best 3D illusion shot a series of it with different lighting ratios. The camera's flash is always in Group A, so set up the slave in Group B. The tone of the shadows doesn't change the impression of the 3D shape much. The tone provides the clue to the environment the objects / faces are in, and by inference the mood of people.
It's not a creative exercise but one that will allow you to better pre-visualize how to create different illusions of 3D shape and simulate "normal" or other than normal (late afternoon) lighting with the vertical angle of the flash and how to create different moods / impressions with lighting ratio.