3 : Render out the scene, and you will have your scenery with shadows moving in a single layer for your final composition.
Characters, shadows and background in one layer
Background and shadows only layer
Save a different file for your characters, this time turning the scenery layers off. Render the character layers and you will have another layer for your final composition.
4 : Composite both layers in AfterEffects or any other similar software, capable of importing sequenced image files and reading the Alpha Channel of the Targa files.
If rendering the shadows and scenery together, is still taking too much time you can render your shadows in a separate layer. To render shadows separately follow the next steps.
Case 2: Shadows - only layer -
To render shadows only
1 : The object that is casting the shadows should have the primary visibility stat unchecked. Maybe also you could uncheck the visible in reflections and visible in refractions stats too.
2 : The object that receives the shadows needs to have the Receive shadows and Primary Visibility stats checked.
3 : Assign a Use Background material to the object that receives the shadows, this material allows you to disappear the object but still receive the shadows.
If you view the alpha channel of the object you will see the shadows.
