the bean chronicles

First of all, fictional characters are imperfect people in that no one is actually able to create a complete replica of a fully-functioning human (with all the different facets of their personality and stuff). A character is meant to be a part of a story, and each character is built for a specific purpose. So often, this makes it very hard to accurately type a character, because not only are they incomplete, they are also often inconsistent. BUT, we can try our best!!

Not every person within a type is the same. But in the case of fictional characters, I find there are a few similar tropes used that make it pretty easy to notice when they are an ISTJ.

The first one is that they almost always have some kind of personal moral code or concept that they follow to the ends of the earth. They take this "moral" very seriously, and a lot of their actions/backstory is oriented around it. If they are faced with a decision where they have to compromise this code, it gives them a lot of conflict. And in the situation where a previously-existing code of theirs gets suddenly disproven, they often are quite rattled by it and can become lost. If someone does something that is the opposite of their code (or conflicts with it), they often get irritated and can be condescending towards said character.

Secondly, they are often very forceful and often ruthless in implementing order (or in some cases, trying to convert people to their personal code). This is a behavior that is common for ESTJs and possibly some other types as well, but a key difference is that said ISTJ character only does it with a select few people that they encounter (i.e. the person they're stuck on an adventure with, or the person they're battling), whereas ESTJs are a lot more open with their correction. A key example of this is Iida Tenya from BNHA, an ESTJ. He is very open about keeping order, to the point of confronting people he doesn't know very well (getting on Bakugou's case about disrespecting the desks in the beginning, and directly telling Deku to stop muttering in front of a crowd of people). And Neji Hyuuga from Naruto, an ISTJ, only really conflicts with the people he's battling and the people in his team. Otherwise, he's pretty reserved. The same can be said for Uryuu Ishida from Bleach as well.

Thirdly, their past experiences (i.e. backstories) are very important and defining for them. This is a common thing for anime characters in general, but ISTJ characters usually take it farther than that, letting past events completely shape how they view the world and other characters.

Fourthly, they are almost always serious and quite reserved (though they are not always quiet).

There are other ways to figure stuff like this out too--in fact, some ISTJ characters you never get to know well enough to see these sides of their personalities, so you have to go off of other things. But for more main characters, these are the things I personally notice the most.