Having recently rewatched BTVS: The Movie I was compelled to compare/contrast what we know in the movie vs. the tv show! There were a lot of things that changed once Buffy was adapted for tv.
BuffyDifferences:
The Movie: Buffy has no last name. Buffy is in her senior year of high school when called to be The Vampire Slayer by Merrick. Hemrey High Stadium is kept in tact after Buffy defeats all the vampires.
Tv Show: Buffy is known as Buffy Summers. Buffy is in her sophomore year of high school when called to be The Vampire Slayer by Merrick. Buffy burns down Hemrey High Stadium in order to kill all the vampires inside.
Similarities:
In both universes Buffy is the most popular girl in her class and is known to be stuck up and basic at school. Both are cheerleaders at Hemerey.
Buffys MomDifferences:The Movie: Buffys mom has no given name in the film but she seems to be very superficial and doesnt seem to be that interested in her daughters life.
Tv show: Joyce is very much the average mom, very caring, and into her daughters life.
Similarities:
Both were married when Buffy was first called to be The Slayer.The WatcherDifferences:The Movie: Merrick has lived thousands of lives with his eternal duty to train the slayer until the day that Vampires no longer walk the Earth.
Tv Show: There is a whole council of Watchers whose lives are devoted to fighting the forces of evil. Since there is a council that has dated back centuries, there is no single watcher that watches over whichever Slayer is activated at the time. We meet Merrick very briefly but even those few seconds you can tell that he is nothing like his movie counterpart.
Similarities:Merrick is the one to tell Buffy that she has been called to be The Chosen One and gives her first round of training.
Vampire SlayersDifferences:The Movie: Slayers are born with a unique mole on a specific part of their body that allows The Watcher to notice them. They also have Vampire Like PMS, whenever a vampire is around they get cramps as their own unique spidey sense
Tv Show: Slayers are not born with any physical marks that are unique to them though The Watchers Council is able to identify Potentials through magic, girls who may be called once the current Slayer is dead. Some are trained since birth while others are not found until after they've been activated. Slayers don't have cramps when Vamps are around, they may be able to feel vamps after they are well acquainted with them but it is very less defined.
SimilaritiesThey both follow the same lingo: Into Every Generation There is a Chosen One... blah blah blah
Both Buffys have prophetic dreams.
VampiresDifferences:
The Movie: Once someone is turned into a vampire their physical features change. They get bat like ears, fangs, and the ability to fly. Once a vampire is slayed, their bodies do not seem to shatter into a poof of dust. They just lay there like a normal corpse.
Tv Show: Vampires in the tv show are able to hide their true nature a lot easier, they keep their normal facial features until they are ready to reveal their true nature in which their force turns more vicious with different colored eyes, fangs, rigid forehead. They are not able to fly. Once slayed, Vampires turn to dust as to not leave any evidence of their bodies.
SimilaritiesBoth have the same weakness to wood and holy water. While it isnt completely clear in the movie if you must drink a vampires blood to turn into a vampire we can assume that both must be near death and drink the vamps blood to be sired.
The Big BadDifferences:The Movie: Lotho is an ancient Vampire who has been killing Vampire Slayers for a millennia. He seems to hunt them down and is just rude.
Tv Show: From what we know in the tv show, Buffy doesnt seem to have had a big bad when she was first called. She just fought a bunch of vamps but once she gets to Sunnydale she faces The Master who is an ancient Vampire who has killed Vampire Slayers throughout history. He was stuck on The Hellmouth and needed The Slayers blood to finally release him.
Similarities:Both Lotho and The Master command a group of vampires and have trusted generals. In the movie it was Amilyn who is a bit of a goof. In the tv show it is Darla and Luke.
Side fun fact:
Ben Affleck had a cameo in the movie! As did Ricki Lake.
Along with Hillary Swank who had a much bigger role as one of Buffys friends.
Seth Green had a cameo that was cut from the film as a vampire.
Source: Me + BTVS The Movie + BTVS Tv show