Charlie Cox is just adorable. That's all there is to it. He was an immediately noteworthy choice for this movie because he perfectly embodies the whimsicality of the Harry Potter universe. His first major leading role was Stardust, the film version of a Neil Gaiman novel that had as interesting a take on magic as Harry Potter. And to be honest, that performance is a big part of the reason why he seems like a perfect fit for the role of Newt Scamander. The average wizard sort of comes off as a lovable middle class eccentric, but doubly so for one who basically spends his young adult life as a shiftless backpacker travelling around the world cataloging cool animals. And one of Charlie Cox's strengths is being able to play a hero, but not the type of person who seems like they would be a hero. Someone who is unassuming, but capable of great feats of magic when necessary. Granted, there's not a ton of info about Newt, but it seems like him in a nutshell.