@NoShip wrote me a PM about my post regarding drafts and shooting dates... I thought I'd put it here, because it's an interesting bit of history regarding the production of BtVS season 1.
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I know Witch was definitely the first episode shot. Welcome to the Hellmouth/The Harvest, I think, were next. But many of the scripts written first were Monsters of the Week very light on continuous plot that had a few scenes of Angel dropped in as bookends or just dialog references in later rewrites.
Casting began in 1995. We know that. The writing of the unaired pilot is dated January 1996. Pilot season is usually around spring. The first season started filming around fall 1996 and there are drafts dated back to August 1996. The first season was pretty much all shot before it ever aired, so the decisions weren't made because of audience reactions as it aired.
Many shows will film what is felt to be a more inconsequential MotW first where the main cast can build chemistry together, so that when they film the first episode(s), it feels more established with any portrayal kinks worked out. Off the top of my head, I can think of Romana II being tweaked a bit in Tom Baker's era of Doctor Who from the first serial shot (where Lalla Ward plays Romana much more like Mary Tamm's Romana I, even down to the costuming choice) vs. the first one aired.
And Witch (dated October 1996) probably benefited from some of those kinks and chemistry having already been worked out from filming the unaired pilot first. By the time of Welcome to the Hellmouth (dated September 1996--written first but filmed later), there's no doubt that Angel had moved up to romantic lead over Xander, no matter their screen time, despite the very opposite being apparent in the unaired pilot where Joss Whedon chose to cut Angel out of his cut-down presentation with Xander being portrayed more viably as a love interest.
Another thing that shows in the alterations is Darla being kept alive (rather than killed by Willow's holy water) and her wardrobe changed to something revealed later as a jealous mockery of Angel following Buffy around and planting a dead body in a locker to scare her off on her first day at school, despite not knowing she was the Slayer. What was essentially a throwaway unnamed vampire (Julie Benz was the test dummy for The Lost Boys-inspired prosthetics) turned into the love interest's sire backstory and was kept on for a few extra episodes as the Master's henchman.
The script that was easily the most rewritten was Never Kill a Boy on the First Date (dated August 1996). Owen has a whole different name and the end scenes take place at a bowling alley instead of the mortuary, never mind all the paralleling of Angel, Owen and Xander as potential love interests. Owen was basically rewritten to be an age and species-appropriate version of Angel down to him liking antique watches and historical poetry (yet more hints that Angel is a vampire even beyond the "Bite me" exchange), which was also contrasted with Xander's juvenile Tweety watch and trying to catch a glimpse of Buffy undressing (which becomes paralleled in 1x07 and Surprise where Angel explicitly has his back turned). But then you have Angel and Owen contrasted when one is focused on Buffy's duties as a Slayer instead of it being a social call, while Owen is portrayed as reckless and liable to get himself killed. Xander and Willow end up supporting Angel in being the voices of duty and responsibility, which contrast them to Owen and why Buffy can allow them to be in on her secret identity. Giles' speech at the end, of course, completes this theme. None of this was really in the early MotW draft.
The Pack (dated September 1996) is more similar to the final product, but with very key additions of Willow and Buffy's dialog at the Bronze about Angel and hyena!Xander's use of Xander's own jealousy over Angel (note that Xander did not have the excuse yet of knowing he was a vampire and it had nothing to do with what happened to Jesse!) in his cruel words to Buffy. Buffy is also wearing Angel's jacket in this episode and it's a very pointed costuming decision. It also adds to the cruelty of what hyena!Xander says to Willow, Xander's own interest in Buffy (perverted into a rape attempt) and his jealousy of Angel that leads into Prophecy Girl. While Angel is not in the episode, the few added references to him and the presence of his various costuming gifts to Buffy from earlier episodes end up having a lot of impact.
David Greenwalt's Teacher's Pet (dated October 1996) is one of the few early drafts of a MotW script where the Angel bookends were there from the start and not merely dropped in later. It's telling that he was pulling for the character to explicitly be Buffy's romantic interest instead of just a messenger months before he wrote the script where it became official. 1x07 (dated all the way to January 1997!) was comparatively written quite late, along with the penultimate episode and the finale it led into (also both January 1997!). Nightmares is another from January 1997, which, although without Angel's presence, is all about setting up Buffy's fears of death at the hands of the Master and being turned into a vampire. These mythology-heavy episodes were the last episodes written and shot. These episodes would've filmed very close to the airing of the first episodes.
It's a good example of how having draft scripts for most of the season will allow for connecting elements to be dropped into the final drafts for even the episodes that aren't arc heavy.