Subject at normal speed but background is sped up

[estebanhco]

I was wondering if anyone is willing to help me by explaining how a common effect that's used in many movies is actually done.

I've seen movies have the effect where there is a single subject in the middle of a shot.
The subject is not really moving but everything else around the subject is in fast motion.

If you don't really know what I'm talking about I'll give an example.

Say that there is a guy in the middle of a camera shot.
And the guy is only looking straight at the camera but isn't moving(that much).
But a large group of people are moving in fast motion around him.

If anyone knows what I'm talking about please try to help me.

 

[Trip.Tucker]

The most obvious way is to have your actor remain still over a long period of time utilizing time lapse, but usually a green/blue screen is used with the actor "acting" in front of the preshot footage. To add a surreal effect and to help the two pieces of footage blend, the actor footage is also sped up but not as much as the background. Typically either the background only or both are timelapsed.