I don't know of an easy way to do this. We have the option of dummy objects receiving shadows but not of casting shadows.

The best I could come up with is to use a Spot Shadow (which is like a spot light that casts shadows instead of shining light) and have it project the shadow of your casting object on your receiving object. Like this:



The idea is that your casting object will be out of view in your render:



Hopefully this will inspire others to come up with something better...:unsure:

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I tried a while back to have a characters shadow acting differently than the character.  I used specific camera angles and chromakeyed everything but the shadow and then layed it over the original scene (with character shadows turned off) but wasn't impressed with the results.