Take the following example of an animation in a webstore.
Say you have to write an Interaction Design document about this. I would imagine it would look something like this (VERY simplified of course):
"As a customer, I can add a wine bottle to my cart, so I can buy it"
- Move the mouse over the wine bottle
- Click the "Add to Cart" button
- Result: Wine bottle is now in the "My bag" section
But where/when do I describe the fact that the bottle will float when the mouse hovers over it, and visually moves to the cart box when "add to cart" is clicked? I don't see it as part of the Story/Case/Mock-up at all.