When Michael Chiklis' wife, Michelle (who suffers from claustrophobia), first saw him in his The Thing outfit, she suffered a panic attack and had to be escorted off the set.
A Fantastic Four movie was filmed in 1994 with the intention to not release the movie to the public. The film's distributor, New Horizons, had the Fantastic Four license for some time and would have lost it if they didn't make a movie promptly. When rumors circulated that Chris Columbus wanted to make a movie version of the comic, New Horizons scrambled to film their own version so they could hold onto the license for a few more years. The actors and most of the crew were unaware that the movie was never to be released, but many began to suspect it when the director made no effort to ensure any level of quality in the film. The 1994 version continues to circulate via illegal bootlegs, similar to the Star Wars Holiday Special.
It has been stated by FF creator Stan Lee that their powers are based on the four elements: Mr. Fantastic's stretching ability is based (loosely) on the malleability of water, the Invisible Woman's power is based invisibility of air, the Thing's rocky appearance is based on earth, and the Human Torch's is obviously based on fire.
There are two Wilhelm screams during the Thing/Dr. Doom fight, one as the car and Doom plough into the bus, one shortly after.
The Thing suit weighed 60 pounds and required six hours to put on. It became so hot that two holes were put in (one on the back of the head, one on the posterior) to allow air conditioning units to cool down the actor inside. Michael Chiklis observed that he spent the entire time either filming or with an air conditioner stuck up his ar*e.
Fantastic Four creator (and Marvel editor-in-chief) Stan Lee has a cameo as Willy Lumpkin, the Four's mailman. To date, it's his only Marvel movie cameo based on an actual comic character.