There's a ton of slice-of-life anime out there, and I'm asked how realistic they are pretty often. So here's my attempt at answering this question.
Environments
Do you know the trend among anime fans of visiting specific real-world places from their favorite series, taking a picture, and posting it side by side with the anime scene? Before coming to Japan, I saw a lot of these comparisons. If you like Makoto Shinkai's work, for example, you probably know that his movies use real-world locations.
In a sense, that's not surprising at all. Western fiction does the same all the time. How many times have you seen the Eiffel Tower or the Golden Gate Bridge in a show? What I find special about Japanese animation, though, is that the places chosen are usually not world-famous monuments. More often than not, they're just a small park in a quiet Tokyo neighborhood, a simple walkway next to the tracks, or a hillside road overlooking the city. Storytelling-wise, nothing requires for these to be real places, the narrative would be the same if they were fictional.
It's not that this never happens in Western fiction. Spider-Man comics, for instance, use plenty of real New York settings. But to me, the reasoning feels different. In Western media, even random spots are often chosen because they're somewhat iconic or exceptional. In anime, "ordinary" places are picked because they capture the feeling of what parks, streets, or train stations are like in general. It's similar to photography seeks out normalcy with just a little je ne sais quoi that makes it invaluable.
The result is that when you walk around Japan, everything feels strangely familiar. Is that level crossing the one from “Kimi no nawa” ? Is that train station the one from “I want to eat your pancreas” ? Especially in your first few months in Japan, you'll constantly realize how real everything in anime looks.
The same goes for sound: the insects, the school bells, the level crossing sirens. When I first arrived in Japan, those details hit me really hard.
That being said, there's a lot of Japan you just never see in anime, so don't take my words the wrong way, Japan scenery isn't only what you see in anime!
Characters
This is where anime veers almost completely into fiction. Even in the most "realistic" anime or drama, characters don't act or talk like real Japanese people. As natural as the environment may feel, the characters are almost always exaggerated.
They usually have very specific ways of speaking. Since Japanese allows for many nuances in word choice and phrasing, writers use speech patterns to give characters personality. That's one of the main reasons people say anime characters don't talk like real people. It's not that their Japanese is grammatically wrong, it's that they're overly stylized or expressive in ways that make them stand apart.
In France, we tend to value characters who act credibly and sound realistic. We want to identify with them, and that's often considered a sign of good writing. But from the conversations I've had in Japan, this isn't as important here. Many people told me they prefer characters they can admire, not ones they identify with, so exaggerated expressiveness feels more appealing than realism.
Conclusion
I feel that there are essentially two very different paradigms at work. In one, credible characters allow you to suspend disbelief and accept extraordinary settings and situations. In the other, the familiarity of the environment allows you to enjoy characters who are completely unreal. Both approaches create an emotional link between the audience and the story, but they rely on different elements to build that connection.
Of course, you can find both approaches in both Western and Japanese fiction. But in Japan, there seems to be a stronger attachment to realistic environments than to realistic characters.
So my short answer is: don't believe what happens in anime, and don't expect human relationships to play out the way they do on screen. But do expect to walk around Japan and spot familiar anime corners, while discovering parts of Japan anime doesn't show you.