in case an actor is not anymore available , how possible and time consuming it is timewise to take the complete figure of an actor in photos or in a video in another scene and put her in After Effects / post production to
another room where he sits on the sofa ? I assume an extra layer of difficulty would be brought by him having to say lines he hasn't said in the previous scene which might disqualify that part as doable. And would all the above fall under cgi or rotoscoping ? thanks
Very hard. You would need to rotoscope the actor, which is very time consuming and not always very effective, depending on the content. You would need to light and shoot the new room to try and match the look and feel of the actor shot. Little things like light direction can really sell such effects. Ultimately, you're unlikely to get a very convincing result, especially if you aren't an extremely experienced After Effects user.