During Chris Claremont's run on the title, he had initially planned for Kitty Pryde to be a recurring cast member, serving as a live-in nanny for Franklin Richards (this took place shortly after Excalibur was cancelled). Then-editor of the X-Men franchise Bob Harras wanted Kitty back in the main X-Men team.
Actor Allusion: More "actor input", actually: Julian McMahon, who plays Dr Doom, suggested that metal staples be used to help stitch the scar at the beginning of his transformation. This comes from McMahon's experience playing a cosmetic surgeon on Nip/Tuck.
Chris Evans wasn't so crazy about working on the film either.
Doing It for the Art: One of Chiklis' requirements for accepting the role of the Thing was that his character would be make-up based, rather than the CGI everyone else had previously assumed. This meant that Chiklis insisted upon hours of application and wearing heavy, uncomfortable makeup, over the option that would have allowed him to earn the same money in significantly less time simply by voice acting. Why? Because Chiklis was a fan who knew that underneath the rock skin, Ben Grimm's still a human being, and this was best shown by having his actor go through the same thing.
It may have also been for the benefit of Alicia, who was standing right next to him,(she's blind).
What Could Have Been: The 2005 film was originally meant to be, in executive producer Chris Columbus' words, "the most epic sitcom ever made", and they were consciously trying to duplicate the formula that had resulted in the utter failure of Batman & Robin — Columbus' reasoning being that while Batman wasn't an appropriate franchise to attempt such a formula, The Fantastic Four was. After a few years in Development Hell, the arrival of Tim Story (who, ironically, was mostly known as a comedy director) and writer Mark Frost finally pushed the film in the right direction.
This feel is best seen in the hilarious breakfast scene and the montage that follows it. The whole thing plays out like an hilarious cross of Leave It to Beaver and The Addams Family.