Quick, easy question

[Flying Spaghetti Monster]

Hey ok, this is a general Trek question but i noticed it watching the teaser for DS9's "Dogs of War" In the background someone beams in and interacts with someone else, and the foreground people are talking. How do they do that? I thought that making someone beaming in means filming on an empty set then having them put the guy in.
Help

 

[Michael]

Motion control?

 

[Flying Spaghetti Monster]

Thought that was only used on models.

I thought that when someone beamed, they had to clear the set, and film it once, then film it the same way again.

How do they do complex shots like the one I described?

 

[BigC]

Like the little girl in the commercial says..."It's the mirrors".