And when you have to animate something like that, you will always ask yourself what it would look like if you just ran the to drawings in reverse, so you wouldn't have to do any fro drawings. Because you are always looking for ways to economize on drawings, right? Unfortunately, the answer is usually no; it won't look good.
When it comes to animating the whole figure, this just about goes without saying. Running the drawings backwards will just look like running the drawings backwards, and not like anything natural at all.
What about a head turn? Yes, it can work, if certain conditions are right.
Here is an example that just came off my disc this week:
This man is looking off to the left of the screen. For a moment, he turns his head to look to screen right, holds, and then returns to his original position exactly. I filmed it with two repeats in this pencil test so that you can observe it carefully.
It works because there is nothing in the head that is subject to drag or follow-through. (If the man had long hair, you would have both drag and follow-through.)
It works because what he sees when he looks to the right does not cause a change in expression or demeanor. (If he saw something that startled him, it is doubtful that he would return to his original pose afterwards.)
It works because the blink in the middle of the move works in either direction.
Yes, and yet I did do something to make the return different from the original move: I added three more inbetweens to the return move, six frames that were just enough to show a slower rate of speed.
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And did you notice a drawing error in this pencil test? I did; the hair on the side plane of his head should look narrower after he turns to screen right, but it doesn't. My bad! as they used to say. But I have now fixed the drawings.