Warhammer 40k AU where the "grimdarkness of the far future" is... just a fictional franchise, that was originally conceived as a cautionary tale against the dangers of bigotry, xenophobia and militarism, but, just like irl, spiraled into an actual franchise. Thanks to technological advancements - they were able to make a ton of shows very quickly.
The actual world is that all races coexist peacefully in the Galaxy. Every single character from canon is actually an actor and the fact that the cast is actually just that (a huge group if actors that work together) lead to some interesting matchups.
Actor!Big E and actor!Malcador are actually long time friends of actor!Eldrad and they are kinda sad their characters and actor!Eldrad's don't have screen time together.
A lot of people think actor!Trazyn doesn't even act - he just continues to be himself on camera. Actor!Orikan hates it, but he can't help but laugh at how he himself starts to act like his character.
Actor!Oltyx and actor!Yenekh are actually married irl. In fact, actor!Oltyx proposed to actor!Yenekh on set after their final scene together. The internet found out about it through actor!Djoseras account (who is actor!Oltyx's irl brother, although actor!Djoseras is actually the younger, unlike his character who is an elder brother), who posted a photo of the proposal with him in the foreground doing a "pointing soyjack" meme. Actor!Oltyx commented on it with "bruh 😭".
Actor!Shadowsun was a long time fan of the show (as in was a total nerd about it) and has a secret AO3 account with a bunch of yaoi fanfics she wrote. She revealed this fact in one of her interviews and ever since then the internet was on the wild hunt for the account.
The 40k IP made a collab with futuristic equivalent of a gacha otome game, where the MC can get and romance Guilliman. Actor!Guilliman and actor!Cawl made a series of videos playing through the collab event (partially as advertisement, partially out of genuine curiosity). The videos consisted of the two either going "yeah, he would say/do that" or "nah, he would not say/do that, he would instead....". The internet suddenly saw a rapid spike in Guilliman x Reader fanfiction.
Actor!Fulgrim was adopted and raised by an Eldar family, so he was REALLY uncomfortable doing Angel Exterminatus. Some changes had to be made to the script because he just refused to go with the original, much brutal version.
Actor!Primarchs did some videos where they would read (PG) fanfictions of their characters. They unknowingly stumbled upon actor!Shadowsun's account and read some of her fics. Actor!Farsight was very confused on why she was behaving very giddy after the videos came out.
Tyranids are actually a (mostly) domesticated species, so all Tyranids on set are either CGI or are real but trained and are just as much of actors as anybody else. They get plenty of treats and pets in-between the scenes for doing such a good job.