The short film below was filmed in real time, with no postprocessing or other manipulation of the captured film:
The CGI effects were projected onto the walls and other white spaces using ordinary projectors; puppetry and other practical effects realized the rest of the vision. I was incredulous at first: projected effects like these depend a lot on the POV of the viewer, and the camera moves around a lot here. But from the making of video I learned that the cameras are tracked using Playstation Move controllers, and the CGI is rendered in real time according to the POV. (I'm an Xbox rather than a PS3 guy myself, so I should point out that the Xbox Kinect has a similar demonstration.) The ultimate effect is pretty convincing! You can see more examples at the link below.
Great Films Fill Rooms: Making Of