I watched the latest Attack on Titan episode (Episode 35: Children if you're avoiding spoilers), which had some really surprising material that I didn't expect they would include. I know a certain amount of changes are made in adaptations, and Attack on Titan has been no different. Though it follows the manga closely in most respects, the series has had to pad on occasion to make sure that the TV run time is fulfilled.
In most cases, one chapter is equivalent to one episode, but sometimes it does not directly translate because one chapter might be very action heavy or it might be very plot heavy. A plot heavy chapter tends to fill an episode. An action one does not, because while two-page spreads are impressive in manga form, they are generally only a few seconds of animation. The series also had to contend with the fact that it needs this particular story arc to last exactly 12 episodes to comprise Season 2. No one wants to start the next story arc and abruptly end when it's just getting started.
As I watched Season 2 I could see the new bits and bobs tucked in. What had been a bubble of dialogue talking about a past event became a full blown flashback in one episode, and that was fine. Season 1 had similarly added anime-only scenes to fill out the run time and the added scenes have been pretty seamless.
Last week's Episode 34 signaled a bit of change though. It added some stuff that's from two story arcs ahead in the manga, but that was mostly okay, because even though the information was first revealed later in the story, the anime presented it as a flashback in one character's mind. It didn't change anyone else's perception of the past event. It does change the audience's perception, but in a small, abbreviated way. In the manga the scene is a fairly extended flashback, but in the anime it's a few seconds to make it clear that someone's death was not the accident it had appeared to be.
I was surprised to see it, but because it changed very little and showed what was going on in the mind of the character who was remembering, I felt that the payoff for using it was warranted.
Episode 35 does something strange though. And this is where the manga spoilers come in. They'll be discussed past this point.
Ymir gets an extended flashback in the middle of Episode 35. A really extended flashback. Episode 35 animates Chapter 47 of the manga. This flashback gives us her history from Chapter 89 of the manga, which is after the reader's awareness of the world has been expanded.
The reason I found her flashback strange is that it gives us our first glimpse of the world outside the walls. We knew that people existed outside of them because that's where Reiner, Bertholdt, and Annie came from, and we'd seen flashbacks of Annie's childhood, but we had no reason to think it was much different from the world inside the walls.
But the flashback raises a lot of questions the anime is not set up to answer. By the time we get Ymir's history in the manga, we already know about the truth of the outside world. We know that humanity flourishes out there and that the people of the walls are essentially backwater hicks that have been left alone for the past hundred years while being surrounded by their own people, who have been transformed into human-eating titans.
Because of this, learning that Ymir was a figurehead of a cult that worshiped her as the original Ymir reborn, was very easy to swallow. We already know who the original Ymir is and the significance she had to the Eldian people. We already know about the treatment of the Eldians as a minority ethnic group in the country of Marley.
The anime doesn't explain the significant of Ymir's name, but does show the cult, Ymir's capture by Marleyan authorities, and even her being taken to the seawall around Paradis. It stops short of showing the injection that transforms her into a titan, but it's clear that she and the others were transformed as punishment for their religious gathering, which opens up a whole can of worms that the anime is not going to address without getting to the truth that's hiding in Eren's basement, which isn't going to happen for another two story arcs after the current one ends.
My problem with including this, aside from the fact none of it will be addressed for another 20 episodes or more, is that we see far too much. While the cultists and Ymir are dressed shabbily and can pass for the same tech as inside the walls, the Marleyans are very distinct in their uniforms. The flashback shows that there are people outside the walls with the power to turn others into titans, and these people are more modern than the ones inside the walls.
And they're organized. We're not talking about small scattered villages as implied when Reiner and Bertholdt talk about returning to their "hometown." Those uniforms are things the soldiers of a nation wear.
When the series finally gets to the basement I don't think it's going to be a surprise for anime-only viewers that humanity is thriving outside the walls because they'll already be able extrapolate that from the Ymir flashback.
While the flashback does give some payoff by providing insight into Ymir's personality that wasn't there before, it does so at the cost of one of the series biggest reveals, and I don't think that's worth it. The worst part is that it feels they pulled the material in for run time, since there are only a few chapters left in the current story arc and the remainder are action-heavy so they need all the filler they can get.