Now you will need to mask out the car. Due to the shadow changes I will use 2 masks for the car, one very tight one to cut out the body of the car and another, feathered mask for the car's shadow. You should now have a masked out car driving through your scene just in time to 'hit' your actor.
When the car hits the actor, we want to hide the original actor from the scene and animate a cut out of them to be shoved out of the scene.We will do this in multiple steps, the first of which will be to cover up the actor in our base footage on car hit.
Duplicate the base footage layer, place this layer directly above and call it something like 'cover up'. Shift it to a time where the area of impact is empty and then use a mask to cut out enough to cover up your actor. Make your cover up layer start exactly when the car hits so it looks like the impact made your actor disappear.
The last thing we need to do is cut out our actor for the last few frames and animate them being shoved out of the scene by the car hit.For this, duplicate your base footage again, call the layer 'hit overlay' (or something to that extent) and move it above your car layer.
Trim it down to start exactly at the moment the cover up layer becomes visible.
Time remap layer
Then, using either masks or the rotobrush tool (if you're using Adobe After Effects CS5 or later), rotoscope out your actor.
Now animate both rotation a position of this overlay so the actor is pushed out of the scene by the car hit.
Finally, let's add some motion blur to the car hit effect to make it look a little bit more realistic.