Frank Darabont's "controversial" ending actually comes directly from Stephen King's source material. Written in first-person, David entertains this notion in his mind as a distant possibility, noting there are three bullets and four people (Dan Miller doesn't make it to the car in the novella), but he ends his journal and leaves it in a restaurant that the survivors have sought refuge in before the car runs out of gas. Darabont felt this ending was too ambiguous and wrote the story to its finite climax, an ending that Darabont says in the DVD commentary was endorsed by King as the ending that King wished he would have thought of.
According to Cinefex magazine, Frank Darabont did not originally plan to include the giant, six-legged behemoth which walks over the car, even though this is one of the novella's most popular scenes. Several CafeFX special effects technicians convinced him to put it in the film.
The original Stephen King novel was also the inspiration for the video game Half-Life (1998), where scientists at a top-secret military base are running experiments with inter-dimensional portals and open the floodgates to its hostile inhabitants.
At the end of the film, when the rescue truck with Melissa McBride passes by David, Frank Darabont originally wanted a second truck to pass by David. This one would have been filled with various people from the market, including Jim, Bud, Mr. Mackey, and most of Mrs. Carmody's ex-followers, indicating that they were rescued safely from the store and making David realize that he and his group should have never have even left the market in the first place. Unfortunately, most of the extras and other actors had already left because their parts were finished, so Darabont had to scrap this idea.
The flamethrower at the end was constructed by the effects department out of parts purchased at Home Depot, which frightened Frank Darabont on all kinds of levels.
Originally, David and his group, while driving away from the market, were supposed to drive by the webbed, desiccated body of the Woman With Kids at Home. However, Melissa McBride's acting during the scene where she leaves the market was praised so much that it was decided to not only have her character survive, but bring her back at the end completely unscathed and with her children.
Frank Darabont originally wrote an opening scene showing the military scientist referenced to by Private Jessup accidentally opening the dimension portal that allows the creatures and the mist to enter our world. Over dinner, Andre Braugher questioned Darabont whether this scene was necessary. After thinking about it for a week, Darabont was convinced to scrap the scene, leaving the nature of the mist more ambiguous.
When Marcia Gay Harden received the script, she was resistant at first, having never done a horror film. She apparently called Braugher to talk about it and he encouraged her to take the role, saying to "View it as an actor's piece and not just a monster movie."
Darabont added the Private Jessup character to drive home the danger that Mrs. Carmody and her mob represent in the vein of "Lord of the Flies" and "The Lottery".
Darabont felt the novella's ending (the survivors drive off into the mist hoping to reach safety) was too open-ended for a film, but contrary to the belief that he simply created this new one himself, the inspiration is right there in King's tale. David in the story thinks to himself that if worse comes to worst, they have three bullets in the gun and four people in the truck. King never has them act on it, but Darabont does. "If we're gonna make a horror movie based on a Stephen King story, let's take Steve's most horrible, dour, and darkest thought and follow it out to its logical conclusion."
Although not directly stated in the movie, it is implied that the creatures entered through an extra-dimensional rift as a result of Project Arrowhead, a secret scientific project being carried out on a nearby military base. An early draft of the script written by Frank Darabont included a prologue set in the base's laboratory, providing a reasonably good idea of what the Arrowhead Project was supposed to have been and what went wrong. In the prologue, a number of civilian scientists, computer technicians, and Army personnel gather around a large object which resembles an old-fashioned diving helmet: a metal tank with thick glass windows. One of the scientists expresses some concern about running an experiment in the middle of a thunderstorm. His superior tells him to relax and orders that the device be turned on. When the machine is activated, a small point of white light (described as looking like a flashlight shining through a keyhole) appears inside the tank. Moments later, however, lightning strikes the base's electrical generator. The point of light begins to get larger and brighter. One of the scientists calls for the machine to be turned off, but a technician tells him "We can't; we're drawing [the power] right out of the sky." The scientists stand helplessly by as the portal inside the tank gets wider and wider and a white mist begins to fill the tank. Something "slithery" is then glimpsed moving inside the tank. A colonel asks the scientists how thick the glass is in the tank's windows. The scientist, sounding none too confident, tells him that it can withstand up to 40 times the pressure of our own atmosphere. Apparently, that isn't strong enough, because the windows of the tank begin to crack and finally shatter outwards, releasing the mist into the laboratory.
It was Jeffrey DeMunn's idea to have McBride's character return at the end aboard the refugee truck with her kids and other survivors.
Darabont's initial script opened with a scene in a military lab showing the accident that ultimately releases the mist, but Andre Braugher convinced him to cut it. "I'm very, very glad I did because I don't think it tonally matched and would have wound up on the cutting room floor anyway." Producer Denise Huth adds that it would have ended up being a "very expensive deleted scene."
Before David and his group leaves, Mrs. Carmody requests that Billy and Amanda be sacrificed. A popular theory by fans is that Mrs. Carmody was right and that Billy's and Amanda's deaths at the end made the mist and monsters go away (given that the mist recedes soon after David kills them).
It is never revealed what happens to Norton, we don't see what happens to them. They walk out into the mist, along with the Biker who tied the rope to his waist and went out with them to get the shotgun. All the focus is on the inside of the store, feeding the rope through the door. The biker gets killed moments later, which leads us to believe that whatever got him also killed Brent Norton and the others. Later, when some of the characters are discussing escaping, they refer to 'ending up like Norton and his group'. So the people in the film believe he was killed. Norton does not reappear again in the film.
Mr Sunday Movies is good Youtube channel if you like retrospective/behind the scenes stuff for movies. Every time a new release in a franchise is coming out they go through the whole series and talk about the movie and get into the behind the scenes dramas and fun facts etc.