The Toei Tokusatsu Fan Club app streamed a round-table discussion with Zero-One's producers, which revealed a lot of information about the show.
Shout out to Tom Constantine for translating this info over on twitter. Planning for Zero-One started in during the summer of 2018 while Build was airing. (this would seem consistent with Gen Urobuchi's comments that his first meetings for Gaim were in summer 2012, with Gaim beginning in October 2013)
Omori said that while Rider was going to change for the Reiwa era, they didn't plan for any huge abrupt changes from previous shows.
Producer Omori visited NII (National Institute of Informatics) and talked with about 10 professors about their feelings on AI.
They also visited AIST (National Institute of Advanced Industrial Science and Technology), where they make robots and advanced 3D printers, in order to research the multidimensional printer 'Zat' that was seen in the show. The producers joked that the place reminded them of Solbrain Base. While they were they, they were shown a Sony Aibo, but they didn't consider putting one in the show at the time.
Auditions began in April 2019, finishing around May.
The first draft of episodes one and two were done on May 1st 2019.
They wanted to start filming at the beginning of June 2019, but were delayed until mid-June.
They didn't expect how much of an impact the comedian HumaGear, Gut-Busting Taro, would have in episode one. The response made them realize that it had been a good choice to pick AI as the theme for the show.
Actor Akio Kaneda, who played Aruto's boss in episode one, would ask to reappear again every time he bumped into a member of the show's staff.
They originally wanted fully animated scenes in episodes five and six (the second episode with the manga assistant HumaGear and the episode with the voice actress HumaGear), but it would have required six months to make them. Filming was in August and the episodes were to air September & October, so they barely had two months to complete the episodes.
They hadn't originally planned to have actor Shinya Owada play himself in episode 11. After Owada was cast they decided to re-write the character so it matched up with the real actor.
Originally they had planned for Gai to take over Hiden Intelligence a lot sooner, with Gai sat in Aruto's office with Izu beside him around the New Year's holiday (around episode 16/17). Eventually they realized it was too quick as people wanted to see Aruto as CEO more.
Omori admits that some fans didn't like the "5 Job Competition" arc, but he loved it and viewing figures grew during that period. He thinks that Zero-One was probably the only show that could pull off this kind of story. They admitted though, that the "5 Job Competition" arc didn't really need to have any Riders appear and the action scenes didn't exactly mesh with the drama portions.
Omori worries that Gai's evil intentions were actually a little too realistic and a character doing workplace harassment on a Sunday morning might have been a little too heavy
Producer Momose based the flower arranging Humagear on his university friend and his mother who were both specialists. They asked them for assistance and by chance they were already fans of Zero-One.
Momose asked his lawyer friend to look over the dialogue for episodes 21 & 22.
For episodes 26 & 27, they went to the Fire and Disaster Management Agency, who train firefighters, for advice.
Gai getting beaten up a bunch later on was a reaction to backlash to the "5 Job Competition" arc.
They were reforming
Metsuboujinrai.net to come back for the show's climax when the show had to go on pause due to the pandemic
They had completed up to episode 35 and written up to episode 39 when the emergency declaration was made. They didn't know when it would be lifted and how many episodes they would get to film.
If Corona hadn't occurred, they would have had 49 episodes total. At one point, it seemed possible that the show would end up with only 41 episodes.
In the end the break cost them four episodes. As they had already written 36-39, this left them with six episodes to wrap everything up. Because of the hold-up, they ended up introducing Ark quicker than planned (in President Special 2).
Aside from Ark-One and ZAIA's new president being introduced, they basically started from scratch with four episodes left. They always wanted Zero-One Vs. Horobi as the final fight (with Takaiwa in Horobi's suit), but weren't sure how to get there. It was in a meeting around that time that someone came up with the idea that Aruto becomes Ark-One after losing Izu and going evil because Horobi killed her.
By the time they finished episode 45, Saber's team had finished filming the first two episodes of their show. (we know the Zero-One cast were receiving flowers at the beginning of August, so I'm guessing that means that the beginning of filming for Saber might have been delayed from the usual June time?)
So did they reshoot the movie to take place after the series then?
¯\_(ツ)_/¯