As an Art director I made many animatics for research and lost many ideas along the way.
It is a cruel and highly flawed process, so to give your ideas the best chance, you need to know a few tricks. (scroll down)
I like to approach an animatic the same way a director would shoot a tv commercial.
First quickly establish your scene and introduce your cast. Keep your camera angles simple and consistent. And then concentrate on making sure your narrative is clear.
Make sure you have enough frames to fill the 30 odd seconds or just as importantly,
make sure you don’t have too many. (10 is too few and 23 is too many)
Creative research is especially good at killing ideas that have a complex narrative or are highly reliant on talent performance. So it sometimes helps to exaggerate the actions and expressions in the illustrations to speed up communication. But probably the hardest thing to get into an animatic is emotion. To overcome that I think it is important that the characters in your ad are likeable. (this is a biggie)
Lastly, don’t rush the client bit. Make sure the product message and the branding are clear, otherwise you will be writing new scripts.
Some of the animatics I've worked on have been meticulously planned by the art director, others I have just been supplied a script. In those cases I will usually start with thumbnails and then plan with the art director the animation the ad needs.