Those shots are actually a combination of :
– Locked off cameras with jitter and motion added in post
– Lots of rotoscoping
– Some greenscreen
Basically you film the actors who are to be cardboarded on a clean plate (no other actors in the shot) and they do their thing on a greenscreen. You take a snapshot of them from the same angle as the camera is in (hell, if its HD you could prolly even take a snapshot with the cam… the cardboard cutouts don’t need to be high-res)
You then produce the cardboard cutouts and place them on set. Before you shoot the scene with the actors and the carboard cutout, you make sure to record a clean plate.
You then film your scene with the actors picking up the cardboard cutout.
In post, you composite the live cardboard actors over the cardboard cutout and then blend them together whenever you make the transition from live action to cardboard action…