You can enable depth of field in your camera, but this will only affect 3D objects that are different distances from your camera. Imagine 3 solids, 50x50 pixels, that are close, near to and far away from the camera. You could change the focus settings to make each of these solids come into focus in turn.
Your video is a 2D layer, though. AE can't just "work out" which areas should be in focus and which shouldn't. Instead forget about the camera completely and add an adjustment layer above the footage. Apply a fast blur to the layer and then draw a mask on this layer around the actors. Feather out the mask so that the blur edge isn't stark. Then you will have to keyframe the mask shape/path property if your actors are moving. It's a tricky thing to get right, so I would advise using it sparingly. If you use a tripod and your actors are relatively immobile though, it can look pretty decent.
Simon Bonner
