This film was originally set to have a much darker tone, essentially poking fun at the original series, and was set for a PG-13 rating. Shaggy was set to be a stoner, Velma and Daphne had a side relationship, and there were many marijuana references. According to Sarah Michelle Gellar, after the cast had signed on, there was a change and the film became more family-friendly.
The director, Raja Gosnell wanted a real life couple to play Daphne and Fred. His first choice was Sarah Michelle Gellar and Freddie Prinze Jr. Prinze Jr. didn't originally want to do the movie because he felt it wouldn't live up to the Scooby cartoons, but Gellar talked him into it.
Sarah Michelle Gellar had to film Scooby-Doo around her already hectic Buffy the Vampire Slayer (1997) schedule. The producers of both productions arranged it so that she would spend two weeks in Los Angeles shooting Buffy, then the next two weeks in Queensland, Australia filming Scooby-Doo.
Frank Welker, who has voiced Freddy Jones in all the Scooby-Doo cartoons, provides his voice to one of the evil creatures in this movie. From 2002 Welker has also taken on the job of voicing Scooby-Doo.
There was an alternate animated opening, showing the cartoon versions of the characters but it was cut for time. This sequence also featured a rendition of the Scooby Doo, Where Are You! (1969) theme by artist Shaggy, part of which appears in the final film.
Originally, Matthew Lillard would scream for a very long time to make his voice all hoarse to do a perfect Shaggy imitation. It didn't work out, so he began trying to imitate the voice he did when his voice was all tired out from all the screaming.
When Daphne attempts to recruit Shaggy and Scooby to inspect the castle with her, Shaggy objects. He says: "Like, Scoob and me don't do castles. Because castles have paintings with eyes that watch you, and suits of armor you think is a statue with a guy inside that follows you every time you turn around!" Everything that Shaggy describes, occurred in Scooby Doo, Where Are You!: What a Night for a Knight (1969), the very first Scooby-Doo cartoon to be aired.
Shaggy is a strict vegetarian in this film. Casey Kasem, Shaggy's original voice actor, insisted that Shaggy become a vegetarian during production of Scooby Doo, Where Are You! (1969), though this was not always the case in various spin-offs.
When Fred, Velma and Daphne meet up at the airport, there was originally a flashback for each of them showing what they had all been doing: Fred on a book tour, Velma in a support group and Daphne learning Martial Arts. The flashbacks were cut out because they were deemed to dragged out the scene.
The idea of a live action Scooby-Doo movie languished in "development hell" throughout most of the 1990s. In 1996, Jim Carrey was attached to play Shaggy and Sara Gilbert was attached to play Velma. At one point, director Kevin Smith was attached, but later dropped out. Later, Mike Myers accepted the project and was the one who most often had his name linked to it (Myers' friend Janeane Garofalo was supposedly tapped by Myers to play Velma). Eventually, even Myers had to leave the project.
The scenes filmed in Australia were filmed at Bond University, in the Gold Coast, which is known to have perfect weather 350 days a year. Unfortunately, Linda Cardellini got a cold, and filming had to be stopped for a day.
It's ironic that Scooby Doo eventually became a live action movie, because the gang featured in the Scooby cartoons is actually based on the real actors/characters from Dobie Gillis, a live action sitcom from the sixties. Fred is based on Dobie, Shaggy is Maynard, Velma is Zelda and Daphne is Thalia.
Neil Fanning was on set every day to provide Scooby's dialogue for the cast to interact with. This led to him and Matthew Lillard becoming close friends, as they had to be very inventive to make Shaggy and Scooby's friendship convincing (since Lillard was acting with nothing there except Fanning's voice off-screen).
When Shaggy and Scooby are approached by the spooky island representative on the beach Shaggy claims that they don't go anywhere with spooky, haunted or abandones in the name, to which Scooby adds 'or rydrocgronic.' And Shaggy says 'Right, or hydrocolonic' which is often misheard as 'or hydrocronic' which hydrocronic is slang for marijuana.
The kitchen in the Spooky Island Castle where Shaggy and Scooby have their burp/fart competition is the same kitchen in which the brainwashing training video was filmed. This explains why there is a camera in the kitchen and Shaggy says "lights, camera, action. Huh Scoob" when he notices it.
Scooby tries to escape the Monsters by hiding in a suitcase and slowly crawling away. When one of the monsters sees him and takes the suitcase off, his tail forms a question mark as if to say "what just happened?"
As the gang arrives on the island, Mondavarious (Rowan Atkinson) says "...It's a dead world after all" - this is the same line (with one word changed) from the song "it's a small world after all" which was briefly sung in 'The Lion King (1994)' by Zazu who is also voiced by Rowan Atkinson.
The trivia items below may give away important plot points.
Originally, Scrappy Doo was not going to be the main villain. At the end of the film, Old Man Smithers aka "the Luna Ghost" from the beginning of the movie was going to be behind the event of the film.
Tim Curry, long term fan of Scooby-Doo, was offered the role of Mondavarious, but turned it down when he heard that Scrappy-Doo - a character he doesn't like - would appear in the film. Curry previously played Ben Ravencroft in the direct-to-video feature Scooby-Doo and the Witch's Ghost (1999).
Throughout the film, there are hints that Mr. Mondavarious is actually Scrappy. He is seen scratching behind his ear as if he had fleas. His office also has bookends in the shape of dog heads and a carved dog head statue.
Unlike the other members of mystery inc Daphne's appearance is noticeably different from her original cartoon counter part. In the film Daphne wears a purple jacket in the beginning of the film. On Spooky island Daphne wears 2 skimpy purple dresses complete with pink go-go boots. Near the end of the film she wears a purple jump suit. Plus Daphne is more muscular as she was able to lift a huge beam while trying to open the door of an abandoned roller coaster in the spooky castle and manages to karate one of Scrappy's henchmen without breaking a sweat. Daphne's personalty is also different as she is slightly tomboyish, sarcastic, fearless, confident, snobbish and conceited making her more of an anti-hero than the rest of the gang.