Say, Beast.. did you get a whole new personality overnight?
In the first two issues of X-MEN in 1963, none of the heroes received much character development. Well, except for Professor X, who was the same imperious, overbearing dictator he always has been. Those issues had to set up the six main characters and their abilities, the premise of two warring groups of super-powered mutants (as well as the series main antognist, Magneto) and also provide lots of action. With the third issue in January 1964, though, some space was taken to depict each X-Man's personality a little bit. Scott was somber and felt burdened by his power, Warren was smug and egotistical, Jean was a blank and so forth. And Hank was reinterpeted most dramatically.
In the first two issues, the Beast is given dialogue for a rough, street-comfortable guy. Kind of a stereotypical trucker driver or construction worker of that era. With his build and apelike poses (often standing with his knuckles on the floor), this no doubt seemed natural enough but either Lee or Kirby (or both) saw that this might make him too much like the Fantastic Four's gruff strongman, the Thing. And Hank McCoy suddenly was the team's intellectual who peppered his sentences with slightly pretentious big words. We see him studying advanced calculus in his room and wearing glasses for the first time (I always liked this touch, that Hank wore glasses not for a secret identity but because he needed them. When in costume, he did without and I wondered if being near-sighted was ever a drawback during action.
There was no Internet or message boards in those days, of course, and the letters page only had room for four or five short letters, so I don't know how fans reacted but they seemed to like that it made the character a bit different than expected, with the contradiction between his primitive appeaance and his educated demeanour. Personally, I think Lee might have derived this trait from Johnny in the Doc Savage pulp series (just as the Fantastic Four had their public headpuarters on the top floor of a Manhattan skyscraper rather than some secret location). Johnny was one of the five aides to the bronze man, and he constantly spoke with excessively obscure words that made his dialogue a pain to read and which annoyed his friends to the extent that he often made an attempt to tone this bad habit down. (And, come to think of it, Johnny also began this trait after the first few stories, originally he spoke like everyone else.) Lee handled Hank McCoy's dialogue better than Johnny fared, though, giving him a vocabulary that was within what most readers could follow.
Whether it was Lee or Kirby who started this change in characterization is a mystery, although that early in the game they were working more closely together than later became the case. It's very possible that they worked out the X-men personalities in a discussion before the issue was drawn.