A very intelligent but irrational person is easy to show, but a rational yet dumb one seems much harder to me. I suppose you could ham-fist it by making them suck at various intellectual challenges - any better ideas?
I thought early Bella from Luminosity did a pretty good job at showing someone rational but with no special cognitive abilities (we're talking about averages, here, not idiots). She just had noticed her limitations and practiced at overcoming them, but that by itself was very good at making her more effective.
One of the simplest author tricks I can come up with is to give your character a thought speed and stick to it. Harry seems to run through ten lines of text in seconds, sometimes, but if you go with, say, your speed reading aloud you can get a reasonable estimate of how long it takes an average person to ponder something. People can mention how they zone out; they can miss opportunities; they can not come up with a good enough solution in time. They can make conscious decisions about what they will and will not think about.
- This thread has run its course. You will find newer threads in the discussion section.
Another discussion thread - the fourth - has reached the (arbitrary?) 500 comments threshold, so it's time for a new thread for Eliezer Yudkowsky's widely-praised Harry Potter fanfic.
Most of the paratext and fan-made resources are listed on Mr. LessWrong's author page. There is also AdeleneDawner's collection of most of the previously-published Author's Notes.
Older threads: one, two, three, four. By tag.
Newer threads are in the Discussion section, starting from Part 6.
Spoiler policy as suggested by Unnamed and approved by Eliezer, me, and at least three other upmodders:
It would also be quite sensible and welcome to continue the practice of declaring at the top of your post which chapters you are about to discuss, especially for newly-published ones, so that people who haven't yet seen them can stop reading in time.