There are several differences in BtVS lore between the movie and the series, some of which are…
The Mark of the Slayer
In the movie, the Slayer bears a mark, a mole on her left collarbone. Buffy says she had ‘that thing’ removed, which may be part of why she was hard to find (it’s not ever really said on the show or in the movie exactly why Buffy slipped past the radar). On the show, there was never any mention of the mark. Buffy’s would have been long gone, of course, but neither Kendra nor Faith appeared to have it either.
The Watchers Council
On the show, The Watchers Council is an important part of Slayer lore. In the movie, though, there is no mention of them. Which doesn’t preclude their existence, but according to Merrick, he has been a Watcher for a hundred lives, reborn each time with the knowledge that it’s his duty to find and train the Slayer. His comments suggest that this duty is his alone, that there’s no such thing as a Council, but that doesn’t explain who finds/trains the Slayers who are born when he’s still in diapers. An alleged early copy of the script mentions that he’s from a small village of Watchers, but since I can’t confirm the script’s origin – and even if genuine, the dialogue didn’t make the final cut – I don’t consider the “Watcher village” to be movie canon.
In the movie, a vampire takes 3 days to rise from the grave, but there’s no such time limit on the series. Sometimes on the show they can rise later that night; other times it seems to be a matter of minutes… whichever the episode’s plot calls for 😉
Whenever a vampire is near, Movie Buffy experiences the joy of cramps. As Merrick explains it, it’s her body’s natural reaction to the unnaturalness of the undead. Sounds logical enough, right? Yeah, well, TV Buffy was lucky enough to skip that part of Joss’s lore.
Killing the Undead
The vampires who are staked in the movie simply fall over dead. On the show, however, Joss & Co. created a cool special effect to make staked vampires ‘poof’ into a cloud of dust. Aside from being really neat to look at, the effect also did away with the need to bury/clean up the bodies.
“Angel, better put on your game face.”
In the movie, vampires don’t have a ‘game face’, per se. As opposed to the TV series, where vampires appear human until they shift into their vampire face to feed or fight, the movie’s vampires are all pale with pointed ears and have prominent fangs that don’t retract at will.
Secret Identity Girl
Merrick tells Buffy that her identity needs to remain a secret because if the vampires know the Slayer’s name, they’ll come hunting for her. I suppose that’s possibly true in the series as well, but the series vamps tended to try to *avoid* running into Buffy… except for the ones who wanted to date her 😉
“It’s just that they don’t photograph *well*.”
When Benny, Pike’s friend who has been turned, steps out of a photo booth, the pictures that develop are blank; the Movie Vamps don’t photograph. We saw several vampires on videotape on BtVS and Angel, though. Also on Angel, Cordelia finds a picture of Angel taken 50 years earlier, and Wesley shows Kate a daguerreotype of Darla.
Buffy’s Athletic Talent
According to the movie, Buffy has a background in gymnastics, which seems to come in handy in her training. The show, however, makes no mention of any gymnastic training. Instead, Buffy used to compete in figure skating. This may or may not be related to her abilities as the Slayer, but SMG’s own experience skating came in handy when Buffy spent an evening at the ice rink with Angel.
“They can fly?!”
When Buffy tells Xander that the vampires could have just gone ‘voom’ when they got clear of the graveyard, he’s panicked to hear that they can fly, but Buffy corrects him. That was a ‘voom’ of the driving sort. Vamps on the series don’t fly. In the movie, though, it’s a different story. Benny hovers in mid-air outside Pike’s window; Amilyn appears next to a cliff dozens of feet off the ground. Cassandra’s attacker approaches from the air, and Lothos floats down to her from the ceiling.
Merrick Meets Buffy
In a Season 2 flashback (Becoming I) we see Merrick approach Buffy in front of her school to tell her she’s the Chosen One. In the movie, however, this conversation takes place in the gym after cheerleading practice. Also in the movie, she was a Senior and dating a guy named Jeffrey at the time, but in the episode, she was still a Freshman and planning to go to the dance with Tyler. Oh, and speaking of the gym…
Buffy Gets Expelled
In the first episode of BtVS, we’re told that Buffy was expelled from Hemery for burning down the gym (oddly enough, the fact that it was full of vampires at the time isn’t seen as much of a mitigating circumstance). In the movie, she did no such thing. Vampires did attack the gym during the big dance, but though much staking ensued, there was very little fire (just one electrocution with major sparks and a clever-but-unsuccessful attempt to burn Lothos’ head). Buffy actually wasn’t expelled in the movie, which ended right after the dance, but it’s always possible she would have been later. It would have to have been for fighting or something similar, though, since the gym was still standing when she left.