That's actually really interesting, @XRD_author ! I'm a native English speaker, so I confess I usually tend to go with a 'gut feeling' on what sounds right, rather than a technical correctness. But I'm glad you brought that up: it seems to me a useful tool to be able to give the reader explicit, solid facts--this is the action he did, concretely. He sat, and that was the end of it. Whereas leaving it open as an incomplete action, if used too often might give the feeling of wobbly-ness, of never finishing. It really shows just how nuanced and complicated the entire process of writing is, and how talented authors who manage to get it right are!
Nah, that's a poor analogy. People who "parrot" 'show don't tell' generally do so with context. It's often used as criticism to a specific person, object, or action in a story that would benefit from being shown to the readers. But even without context it's fine. I don't require context to understand the advice 'dress for success' doesn't mean I should never wear casual clothing in public. And neither does the overwhelming majority of people.
I think there's still some misunderstanding here. We have two possible situations that we may want to describe: 1) The transition of a person from a non-seated position to a seated position. 2) A situation in which a person is already in a seated position and remains that way. I would say that (1) is most often described with a simple-past "he sat" and (2) is most often described with a past-continuous "he was sitting". It could also be described with the somewhat more formal and less ambiguous "he was seated". (And, no, that isn't passive voice. I'm not referring to someone else seating him.) I think that you had gotten the impression that "he was sitting" is used for (1)--the transition to a seated position. But that's not how it's usually used.
Ack! I totally tunnel visioned! I was using both "he sat" and "he was sitting" to describe (2), and completely neglected the obvious fact that 'he sat' could clearly be used for (1) as well! As in "Gandalf sat in a corner, his brimmed hat pulled low, smoking a pipe." Words is tough... Thanks @ChickenFreak !
But that's not always a bad thing. I agree that the two different phrasings serve different purposes, but not that one is definitely better than the other. It depends on the purpose.
I think that's the key to almost everything writing-related. Not better, not worse, just more or less effective at the current task.
