What is more difficult is telling a new short story using only footage from a show(s) and making it look like it was filmed that way. The video below is one of the best I've seen. The time it must have taken to place two characters on two different shows together on screen and look almost seamless is astonishing.
WHOLOCK is a great short film by John Smith that every Whovian and Sherlock fan should watch. Even if you're not a fan, check it out for the fantastic effects. Then check out the breakdown for all the effects work done for the short.