This has shifted my perceptions of what is in the wild significantly. Thanks for the heads up.

At the edge of my vision, a wiggling spoon reflected the light in a particular way. And for a split second my brain told me “it’s probably an insect”. I immediately looked closer and understood that it was a wiggling spoon. While it hasn’t happened since, it changed my intuition about hallucinations.

This matches my own experience with sleep deprivation in principle. When I have been severely sleep-deprived (sober; I don't drink and don't use drugs), my brain has started overreacting to motion. Something moving slightly in my peripheral vision caught my attention as if it were moving dramatically. This even happened with stationary objects that appeared to move as I shifted position. I have experienced about a dozen such false positives in my life and interpreted the motion as an insect only a couple of times. Most times it didn't seem like anything in particular, just movement that demanded attention. However, "insect" seems an obvious interpretation when you suddenly notice small rapid motion in your peripheral vision. ("Suddenly" and "rapid" because your motion detection is exaggerated.) In reality, it was things like wind gently flapping a curtain.

However, this is not the only way people can hallucinate insects. There is another where they seem to see them clearly. Here is Wikipedia on delirium tremens:

Other common symptoms include intense perceptual disturbance such as visions or feelings of insects, snakes, or rats. These may be hallucinations or illusions related to the environment, e.g., patterns on the wallpaper or in the peripheral vision that the patient falsely perceives as a resemblance to the morphology of an insect, and are also associated with tactile hallucinations such as sensations of something crawling on the subject—a phenomenon known as formication. Delirium tremens usually includes feelings of "impending doom". Anxiety and expecting imminent death are common DT symptoms.

From this and a few articles I have read over the years, I get a sense that when people are suffering from delirium tremens, they see small creatures of different types distinctly and vividly. So you can probably say there are "insect hallucinations" and "Huh? Is that motion an insect?" hallucinations.

Hallucinations can be very realistic. My experiences with hypnagogia taught me that hallucinations are coming from the same brain that paints the real world for you, and it can paint hallucinations as realistically as anything else. But their quality will depend on what's causing them. Probably the most common reason for hallucinations is pareidolia operating on  low detail information gathered from your visual periphery. These vanish when you look at them.

But there are other ways to generate hallucinations. I've also experienced hallucinations from sleep deprivation. These were less realistic and might have still been generated by pareidolia. These appeared as I looked at them, but they faded away as I watched them. If you want an example, looking out the window from the passenger seat I saw a jogger wearing bright neon clothes. They were moving very slowly as if they were floating just above the ground. This illusion faded and I saw I was actually looking at a reflector post.

It has been theorized that enough people experienced hallucinations induced by intense grief following the death of loved ones, that it inspired the ghost phenomenon. It isn't terribly uncommon for nursing home patients to see hallucinations, including ghosts, and there are prescription medications which are linked to this issue. It's hard to tell what things look like for them, but they often report seeing things they believe to be real. So movie depictions may be justified in depicting hallucinations as crisp and realistic, although not all hallucinations fit that category.

In my experience, that's pretty much what 5-HT2A agonists (hallucinogens) do but to a stronger extent: You see peripherally a curled leaf on the ground, and perceive it as a snake before you take a closer look, or you see patterns on a marbled tile, and the exact positions of the shapes slowly wobble.

My understanding is that this is because you assign a lower confidence to your visual inputs than usual, and a higher confidence to your priors / the part of your brain that in-paints visual details for you.

I'm really confused, we must not be watching the same films or television because almost by virtue of being a hallucination scene it is inherently depicted as different, or more stylized than the rest of the film as a way of telegraphing to the audience that what they are watching is a hallucination and not real. Not realistic, in fact they often make a point of making them less "realistic" than the surrounding film.

Crisp? Depends on what you consider crisp - the negative space and white in Miss Cartiledge's scene certainly makes the colours "pop" more. But what about this scene from Requim for a Dream which makes use of much shadow and theatrical stage lights, or this Bob Fosse hallucination - once again minimalism (which is inherently unrealistic) makes things "pop" more, but it is very theatrical.

I don't claim this is a cross section. If anything my idea of a Hollywood Hallucination is one of those "way out groovy colours" bad optical print effects that looks like WIlly Wonka's boat ride. But I assume you're not talking about cheesy cliches.

If you were to start yearning for children, you would either (a) be able to resist the yearning, or (b) be unable to resist the yearning and choose to have kids. In case (a), resisting might be emotionally unpleasant, but I don't think it's worth being "terrified of". In case (b), you might be misunderstanding your terminal goals, or else the approximation that all of the squishy stuff that comprises your brain can be modeled as a rational agent pursuing some set of terminal goals breaks down.