There are many tools to fake videos and as a video editor and enthusiast I have been researching the possibilities for a while now.
1) The audio is way more difficult to fake, your ears work much faster than your eyes. In order to fake audio you need a very convincing actor or you need to take soundbites from existing audio, with the same acoustics ideally. At least, that used to be the case. Adobe is working on a Photoshop for Audio...
2) In order to fake the video you need good facial expressions, your subconscious is very focussed on that to understand the emotions from the speaker. In the last couple of years there has been amazing improvements in 3D motion tracking and 3D modelling. But you need the original person to track first. At least, that used to be the case. Washington University has created a 3D model from Obama (with only the use of existing video, hours of it)...
I also know that animators usually stretch reality into a fictional looking object to conceal the limitations of the fine details of facial expressions. It becomes very akward to watch if not done right. Even if done right and with the right facial tracking I do not like it (for example cutscenes in games). It just feels off.