When exporting an animation, Revit calculates the first frame before asking about the compression codec. this can mean you have to wait for quite some time before you can leave an animation unattended. I think it would be much better to make this part of the save dialog instead of a later dialog.

Also, it appears that if Revit doesn't like the Codec chosen, it will only animate 1 or 2 frames and then stop without any notification that it didn't complete the rendering. The Codec that causes this failure for me is Cinepak by Radius.