I was listening to the latest episode of the writing craft podcast Hide and Create on the way home from work today -- the episode is about writing characters who are smarter than the person writing them. Here's the link -- content warning for casual ableism. I have thoughts on this topic, and I was going to write them a response, but I just realized there are 15 minutes left in the episode and I wanted to write it now, so I'm going to write this as an independent post instead, in case the rest of the episode covers exactly these points.
I think that intelligence -- in the sense that they discuss in the podcast, the fancy, mystery-solving, quick-witted, Sherlock kind of intelligence -- isn't a matter of any innate biological or developmental quality. I think it's a matter of values.
First of all, obviously, some characters value having good information more than others. Beyond that, though, some characters value different systems of information gathering than others. Most characters believe they're generally right about things, but if your character's system for verifying information is "That sounds about right" or "It stands to reason' or "My buddy Matt says so," their information is probably going to be wrong more often than someone whose system is "I've read about it from multiple sources" or "Hang on, I'm not sure. Let me check."
I think an important thing to know about your character is, if they encounter a situation they're unfamiliar with, and then have sixteen hours off-screen, when they come back, have they looked it up on Wikipedia? One of my favorite smart-person lines is in the Avengers, when someone asks Stark when he suddenly became an expert on gamma radiation, and he said, "Last night -- am I the only one who did the reading?"
The cast of Hide and Create struggled a bit to figure out how to discuss the other end of the spectrum. I think that the other end of the spectrum -- if the literary manifestation of conventional intelligence is values -- is prejudice and willful ignorance.
Characters who consistently act on false presumptions about the way the world works, about how other people feel or behave, are going to find things working out in ways they didn't plan for, and are going to look like asses while they do it.
Other values -- like pride, preventing someone from admitting they're wrong to correct their behavior -- can also contribute to a character's failure to represent the conventional ideal of intelligence.
Where I left off listening, they were taking the idea of a character who is not intelligent in different directions -- comic characters, like Joey from Friends, or virtuous but not intelligent characters, like Sam from Lord of the Rings. But I think that both of those characters' relationship to intelligence, perceptiveness, curiosity and openness can be better evaluated in terms of their values than in terms of some kind of innate INT stat.
It's been a while since I've either seen the movies or read the books, but if I recall correctly, Sam knew a ton of stuff. Joey's priorities didn't line up with the markers of conventional intelligence in Friends, like Ross's did (I hate Ross so much don't even get me started) but he was perceptive of his friends' emotions and caring towards them, like Ross definitely wasn't. I don't think that difference is a matter of different qualities of brain, I think it's a matter of different value systems.
Maybe there's some amount of talent and innate preference involved in what a person's strengths are, but I think that influence is much, much smaller than what kind of values a character holds and how they apply them to their everyday practice of life.