Well, you could do:
-Precompose your Keyed guy.
-Duplicate him.
-Apply the effect Perspective > Drop Shadow the duplicated layer.
-Make the duplicated layer a 3D Layer.
-In the effects window of the duplicated layer, turn on “Only Shadow”.
– Then, just play with the X and Y axis of, you guessed it, your 3D Layer so it looks like his shadow’s on the ground.
Play with the feather, opacity and distance and the scale of the 3D Layer (shadow) itself.
Hope that helped.
Justin Productions
Tangerin01@hotmail.com
Adobe After Effects 6.0 Professional