Creature effects supervisor Tom Woodruff Jr. played the gorilla Isabel, similar to the alien roles he played in the movies Aliens (1986), Alien³ (1992) and Alien: Resurrection (1997). To obtain a picture in the heat-sensitive infrared light, crew members warmed up the fur of the gorilla suit with hair dryers.
Most special effects shots involved up to seven different plates. In most cases one plate consisted of an empty shot of the scene background where the movements of an invisible Kevin Bacon were later matched.
When Caine injects himself with the fluid to make him invisible, you see bubbles going through the tube into his arm. These bubbles, once in his veins, would cause severe pain (though are probably not enough to kill him). Hence doctors in real life flick the bubbles to the top of syringe and squirt them out. At the least, a medical professional injecting air would be highly unprofessional.
At the start, when Linda tells Matthew that they have to get to the lab, she is wearing a nightshirt and obviously no bra. However, when she sits down at the dressing table, bra straps suddenly become visible under the shirt.
What are the differences between the Theatrical Version and the Director's Cut?
Being released as a Director's Cut nearly seven minutes of new footage were added to this movie. Most of these new scenes are story extensions but there are also two shots with added violence plus the rape scene was prolonged as well. A detailed comparison between both versions with pictures can be found here.
Was this based on a book?
This film is inspired by the novella "The Invisible Man" by H. G. Wells. The stories are quite different though, taking place in a different time period and a different place with a completely different series of events.
That being said, the basic premise of the movie is very similar to that of the book. A young, brilliant, but emotionally unstable scientist invent a way of becoming invisible. He tries it on an animal and then on himself, but can't become visible again (for different reasons though). Eventually this turns him crazy and evil, he suffers from delusion of grandeur and murderous rage. The protagonists have no choice but to kill him in self-defense. Certain small details are borrowed from the book, like using makeup and falling water to break invisibility (although in the book these methods are only discussed), and the fact that body becomes visible again after death.