Not having Carrara yet, I'm not sure I can answer directly, but I'll throw a couple things I do know from RDS and film. If the screen is a distinct object (cube, curved face, whatever), map the movie onto it in its shader. I've used a combo of the color and glow channels to simulate a TV screen for a virtual set recently and it worked out really well. (Be sure you set the mapping mode to cubic, single face.) I don't think this is actually "rotoscoping," though, unless the Carrara manual has taken liberties with a term that's been in use in the film industry for close to a hundred years. Rotoscoping is the manipulation of masks or mattes, usually by hand, with the intent of altering the source image. Whether this is to hand draw a mask to remove an object from its background to insert it into another background, or to use film as a guide for hand-drawn cartoons (think of the old A-Ha video, "Take On Me"), or to convert video images into something else that looks and feels hand drawn (whatever that new Linklater film is called), "rotoscoping" refers to the process. OK, I'm taking my old "teacher" hat off.... (I used to teach kids ranging from age 5 how to do video production; I still sometimes take that same tone. No offense intended.)