Like a bunch of toddlers in an enclosed space.
What's uncanny about him? Simon Baron-Cohen is a world-renowned scientist who was brought in to weigh on Fallon's premise that he's a "psychopath," which is the topic at the center of the books he's selling. As Simon pointed out, Fallon should have at the least got another brain scan done before he published a whole book on some rather tenuous claims. He could have done other things too, but that's a whole other discussion. Fallon, as a practicing neuroscientist, should be firmly aware of these things. And what is the first thing he says when Baron-Cohen questions him? That he became too fat to fit in the scanner to do a second MRI. Right.
There are some interesting layers to the interactions the people on that show had. Firstly, James Fallon is probably actually a "psychopath," and for people who understand the mechanics of manipulation, it is glaringly obvious how he reframes conversations. At one point he even said offhandedly, "I'm creating a story about something we really don't know about." James has to walk a careful line when talking about his "psychopathy" on television. If he underplays it, people will wonder why he wrote a book that basically claimed that he is one. If he overplays it, he'll make himself look like a terrible person, and will probably wreck his career. Hence when the host rather obscenely asks him if he'd feel anything if he killed his own kids, he is naturally obligated to say he would feel something, even if he knew he wouldn't. So many times did he have to smooth things over because they asked him questions a sensible person just can't be honest about.
The dynamic with Simon Baron-Cohen and James Fallon was interesting because Baron-Cohen is by far a better scientist, and does not get impressed by how Fallon cherry-picks information and runs a story with it that supports the notion that he's a "psychopath." I suspect that Baron-Cohen simply dislikes Fallon and what he's doing. Several times he prods Fallon. Although Fallon maintains that people can't tell when he gets angry, I'm fairly certain I seen anger and frustration in Fallon's face, especially when Simon said that Fallon's story was more suggestive of Asperger Syndrome. Of course Simon did not actually think that, but it flipped the focus of the show, and apparently frustrated James enough that he brought it up at the closing. Fallon rubbed Baron-Cohen the wrong way when he tried to drown him out right at the start of the show and couldn't give straight answers to questions. So then Baron-Cohen was mainly messing Fallon after that point, rather comically in my opinion, and made this Fallon interview something more than another grandiose spiel.