One of my pet peeves, in a lot of sci fi or fantasy stories, is when characters exposit about information in a way that doesn't fit their characterization. Specifically, when they know more than they should.
A lot of people might think it's a given - if you're personally affected by something, you'll want to understand it. People like that would be surprised to hear that I know more about cerebral palsy (in terms of scientific knowledge) than my best friend who has that condition. She's a smart person, a university student who gets decent marks, but it's me, not her, who can name the part of the brain that is usually affected in CP, can describe the differences between CP subtypes, and can list many potential causes of CP. Of course, she knows way more than I do about how it feels to move with CP, and what it's like to grow up using a wheelchair, and so forth, but when it comes to the science of the condition, having CP doesn't guarantee knowledge about it. The same is true of superpowers (and other supernatural or science fictiony conditions, but I'll focus on superpowers here.)
A far better predictor of whether you'll know about a condition is whether you have experience with the relevant scientific field. Bruce Banner obviously will know a lot about gamma radiation, given that it was his job
before he started turning into the Hulk. Even Spiderman, who in most versions of the story was some form of student learning about the scientific field (which varies between continuities) that resulted in the mutation-inducing spider, has a pretty good reason to be knowledgeable about it. Similarly, in real life, an medical doctor who get sick will (unless their condition causes enough cognitive impairment to obliterate the benefit of their training) be pretty knowledgeable about what's happening to them. Similarly, an audiologist who develops hearing problems will be well-equipped to understand what's wrong and how to treat it.
Now, just because they weren't an expert in the area before it affected them does not mean they can't become an expert afterwards, and being personally affected can provide a significant motive to learn about something. But not everyone who is personally affected by something becomes knowledgeable about it, for three reasons:
Obviously, a smarter person is more able to absorb information than a dumber person. Most people can readily see that someone with Down Syndrome is unlikely to be able to explain what a trisomy is or how nondisjunction works. Even someone with an IQ in the lower end of average might struggle with highly technical concepts like that.
In addition, because of skill scatter, someone who is highly capable in one area may not be very capable in another. A gifted linguist could nevertheless lack the mathematical ability needed to explain particle physics, for example. Therefore, they may not be capable of understanding how their ability to shapeshift depends on manipulating the strong nuclear force.
This usually isn't too hard for writers to get, so I won't dwell on it too much, but I have seen a few gaffes of this type.
This is the main reason why a person affected by the condition is more likely than a randomly selected person to be knowledgeable about the science behind this condition. But having a condition doesn't necessarily mean you want to learn about it.
For one thing, people cope with life-changing events in different ways. Some people decide to seek scientific knowledge when they acquire a medical condition. Others decide to talk to God, or philosophize about the meaning of life. Some decide they're going to focus on other things, like their family or their work. And some actively deny and avoid information, and try to pretend it isn't happening.
And that's with an acquired condition. If it has always been part of their life, it may never be a big enough deal to bother researching it. (Not everyone researches things for fun, like I do.) They may have learnt what they need to know at a very young age, and not feel a big need to learn more. They're much less likely than people with acquired conditions to invest a great deal of emotion (in any form) into their condition. Someone like that either needs to enjoy this kind of knowledge for its own sake, or to have some pressing motivation (besides just having the condition) to seek out information about it.
Therefore, the character who 'just wants to lead a normal life' is probably not going to be very knowledgeable about their superpowers. Neither will the one who cares more about being a superhero than learning about science. The one who is scared about adverse effects of their powers could go in either direction - either 'head in the sand' or learning as much as possible. The one who has always had powers will have had much more opportunity to learn about those powers, but also less interest in them than someone who's just gained them.
Interest also impacts what kind of information they gain. Imagine a family of hereditary shapeshifters - the shapeshifter with an interest in genetics might learn about how this characteristic is inherited, meanwhile the one with an interest in physics puzzles over where the extra matter goes when a 160 pound human becomes a 90 pound wolf. And the one interested in physiology examines how the body knows how to transform a plantigrade foot into a digitigrade foot, while his psychologist brother examines the emotional triggers that set off an uncontrolled transformation. (OK, now I want to write this family. This would make a neat story.)
Even if you are capable of understanding information, and are motivated to seek it out, you still have to actually find it. If the information you want doesn't actually exist, then unless you can discover it yourself, you won't know it. If you're a brand new kind of vampire, you won't know what your weaknesses are unless you encounter them and survive. If your child has just been diagnosed with a new syndrome that has been described in only one other journal article describing 3 patients, the oldest of which was 10 years old, you won't know if your kid has a 95% chance of developing cancer in his thirties.
And even if the information is known by someone, doesn't mean you can find it. Especially in stories where there's a Masquerade in place - a guy who was bitten by a werewolf he never saw again, in a setting where the existence of werewolves is a closely kept secret, will have a lot of trouble getting information about the condition. Even if the information isn't deliberately kept secret, you may not know what some tribespeople across the world from you consider common knowledge. Or maybe someone knew something, but that information has since been forgotten. If you're the first X in five thousand years, it could be tricky to figure out what the ancients knew.
Furthermore, the information could be wrong
, either deliberately or unintentionally. Maybe a page from that ancient manuscript was torn out, or the author was unaware of something. Maybe the information is out of date, and new knowledge would change the situation. A great example is in one episode of Buffy the Vampire Slayer
, where they face an ancient evil that was sealed away and is now being released. The information they can find on this creature indicates that no weapon created by humans can harm him, but Buffy and friends realize this was written prior to the development of modern weaponry, and kill the villain with a rocket launcher.
Opportunity also refers to your opportunity to gain the necessary prerequisite knowledge and cultural worldview. The book The Spirit Catches You and You Fall Down is a nonfiction story of a Hmong refugee family with a daughter with a severe form of epilepsy. A language barrier combined with significant differences in cultural worldview resulted in very little of the doctors' knowledge of epilepsy being communicated to the parents. This contributed to the girl's progressive deterioration, because her parents did not understand enough of Western medicine to give her effective treatment. For example, they did not know the difference between seizure medications and antibiotics, and did not realize they needed to refill her prescription when it ran out. Both linguistic and cultural barriers can come into play.
In fiction, one specific group for whom culture needs to be taken more into account are long-lived characters, such as vampires. Even if the vampire is adept at blending into human society, the impact of growing up in Medieval France in the 1300s will never completely disappear. It will shape their beliefs and way of thinking, and will have an impact on how they understand vampirism. For characters who were sealed away or otherwise unable to benefit from continuous experience with a changing society, the impact of their time period of origin will be felt even more strongly.
Don't be afraid to leave out some exposition if it doesn't fit in the mouths of any of your characters, or hint at it indirectly. If you absolutely must communicate it somehow, you can always try some non-canon medium such as an appendix at the end. You can tell your audience what they need to know for the plot without getting into all the science behind it. If your superhero just knows they can fly and turn invisible without having a clue how it works, that's fine. Even if you know how their powers work, they don't need to.
And don't be afraid to have your character be wrong about the facts. Even if they never learn any different. If it fits your story for them to be mistaken or uninformed, go ahead and write them that way. Just because a character claims something works a certain way doesn't have to mean they're right about it.
Don't use exposition solely to tell your audience how something works. Tell them something about the character, too. If one person calls vampirism a virus and the other calls it a (literal) curse, that tells you something about how they see the world. And if the 'dumb jock' suddenly explains how magic is the ability to violate the 'matter cannot be created or destroyed' principle, that should tell you about hidden depths in that character, and not just the author's determination to get that information across to the audience.