That's more realistic than some I've seen but it reminds me more of Paul Rudd from his body language.
Wouldn't have even thought that looked like him, still have trouble seeing it.
I am starting to wonder if this concept simply works better for some viewers than others. I seem to always very deeply notice that the shape of the head is always sort of wrong... I'm wondering if I use "shape of head and typical hairstyle" as a major way I recognize people. Maybe other people are really focussing on facial features like "lips eyes and nose"? Honestly, what I keep thinking when I see stuff like this is "this just looks like Paul Rudd wearing a lot of makeup" not "this looks like Jimmy Fallon with a narrower head and a different hairstyle".
Definitely not perfect, but this is hobbyist grade work. With the recent work around parallelized WaveNet synthesizing 10 seconds of audio for every second of wall clock time, a live fake that fools 50% of regular people is probably a couple of years away at most. Particularly if you can control the setting to ensure lighting/angles/etc match up reasonably well.