The Actuality-Exposition Structure
Tal took a casual gander through his spyglass. Below him, over hoof prints scoured by the shifting sands of time, a desert rat was scampering. It was busy with the day-to-day shenanigans of his dismal life. The little critter was scurrying across a crust of earth parched and cracked by a pitiless noon sun, on the edge of a weather-beaten trail that wound through arid scrub. They were the sort of barren wastes all too bloody common in the Norgolian Empire. The rat stopped, sniffed the air and... quick-as-a-flash scarpered for safety, because dust swirled up in blinding clouds at the hooves of three mounts galloping, whirling, rearing. On those horses, three riders were engaged in a good old two-against-one.
Well now, thought Tal. High on his rocky ledge of cliff, he lowered the spyglass, unslung the longbow from his shoulder, reached for an arrow. But stopped because a hand gripped his wrist. Black Raq Skarrion -- who was the king of thieves, notorious in all Norgolia -- rose up from a crouch behind him, arched an eyebrow at Tal's impetuosity. Tal gave him his best mock-innocent shrug. He was a rogue, after all.
Now let's mark that up with what each clause is doing in narrative terms:
Tal took a casual gander through his spyglass [action]. Below him, over hoof prints scoured by the shifting sands of time, a desert rat was scampering [action/activity]. It was busy with the day-to-day shenanigans of his dismal life [exposition]. The little critter was scurrying across a crust of earth parched and cracked by a pitiless noon sun, on the edge of a weather-beaten trail that wound through arid scrub [activity/exposition]. They were the sort of barren wastes all too bloody common in the Norgolian Empire [exposition]. The rat stopped, sniffed the air and... quick-as-a-flash scarpered for safety [activity], because dust swirled up in blinding clouds at the hooves of three mounts galloping, whirling, rearing [exposition]. On those horses, three riders were engaged in a good old two-against-one [activity].
Well now, thought Tal [action]. High on his rocky ledge of cliff, he lowered the spyglass, unslung the longbow from his shoulder, reached for an arrow [action]. But stopped [action] because a hand gripped his wrist [exposition]. Black Raq Skarrion -- who was the king of thieves, notorious in all Norgolia [exposition] -- rose up from a crouch behind him, arched an eyebrow at Tal's impetuosity [action]. Tal gave him his best mock-innocent shrug [action]. He was a rogue, after all [exposition].
It all starts fine, with the first sentence perfectly functional as action. With the second sentence however, action starts to get shaky, as it becomes a little ambiguous as to whether "below him" refers to where Tal's looking or at his feet. We've got an actuality, but is it one that Tal's even knows about let alone cares about? If it's taking place at his feet, that rat's scampering is activity, not action. We're not sure, but it's pushed toward activity also by the past progressive tense, which makes it a description of state as much as a narration of a deed: the rat did not do X; it was in the state of doing X. Things are already not looking so good.
It's with the third sentence that things really go off the rails though. Here, the continuity of action is broken completely, the narrative put on hold as we have exposition in place of actuality, an explanation of what the rat was up to that it should be in such a state of scampering. Because of that exposition, the fourth sentence almost certainly reads as activity; even if we do think Tal is watching the rat, the sense of omniscient distance to the exposition has deflated any sense of import to that. And again past progressive tense makes this a description of the rat's state: it didn't act in that sentence either; again, it was in the state of carrying out a particular activity. (Which we already know, so this is going to read as redundancy too. Awesome.) Worse, we can even read this inconsequential activity as exposition too, an elucidation of where specifically it was doing its scurrying. And with sentence five we get another clear-cut exposition sentence. The free indirect thought here doesn't even work now; it reads more like editorialising than Tal's attitude, feels like it's missing a "Tal considered" or somesuch between "barren wastes" and "all too common" in order to specify that it's Tal's opinion.
The deadening effect is also, like much in narrative, cumulative; once we start to feel like every other statement is not stuff-happening but rather an explication of the what or why, the where and when or how, all it can take is a "because" to ruin a perfectly good bit of action, turn it into a tedious explanation of action. Fall into a pattern of repetition, and you have the deadly, dreary Actuality-Exposition Structure that will suck your reader's will to live: X did Y; Y was a Z; A did B; B was C. He opened the door. He needed his glasses. He entered the room. It was a big room. Blah blah fucking blah. (C.f. the repeated subject-fronting mentioned in How to Write a Paragraph. The two often go together in the type of writing I label deposition.)
It doesn't really matter how you dress it up, the Actuality-Exposition Structure smashes the sense of action by halting the flow every other sentence or clause. It doesn't even matter if it's not strictly speaking every other sentence, if you do actually have a good few action sentences in a row here and there -- as with the start of paragraph two in this example. You need to get the action rolling, build up speed, achieve lift-off, and sustain flight from there on. In that second paragraph, we're not even really in the air before the exposition clause brings us crashing down. Result? None of that passage ever truly achieves the status of effective action. Action is, one might well say, not written in the sentences at all but rather conjured by them. If the conjuring is not sustained, the action is no more there than the melody when a would-be guitarist plays a couple of chords, pauses to adjust their fingers, plays another couple of chords, pauses to adjust their fingers, and so on.
Description can be a sneaky bastard to watch out for here, it's worth noting. As I said in the previous post, description isn't bad in and of itself but it isn't action. And it can all too easily serve as the expository part of an ongoing Actuality-Exposition Structure, as the writer tells us e.g. of a character entering a scene, then what they look like or what the setting looks like, then how they pick up an object, then how that object feels, and so on, in a repeating pattern of dynamic and static lines that will make me want to gouge my eyes out with a spork at its infernal fucking halting effect.
Sure, I'll happily sit back as you pause the action for a rich descriptive passage, a visual tableaux of accomplished conjuring, but if it's a passage of action, you want to be asking yourself if your descriptive detailing can be done inline, within an action sentence -- e.g. describing that character as they enter, weaving the qualities of the object through the narrative of their handling it. Every example that's not inline, every sentence or two here or there which is and only is a flat description... you want to be interrogating that ruthlessly, asking yourself if this is a failure of agility, an inability to carry description in one hand, action in the other, an inability to juggle them so they soar over and under each other throughout.
If you're making two trips to get the character into the scene -- one to walk him in the door, then another to bring in the description of him -- you need to be asking yourself if that's good enough. It probably isn't; it's all too likely deflating your conjuring. That character description? Likely as not, it's going to read as expositional character profile, regurgitated straight from your notes and dead on the page. The actuality will be less vividly conjured too with the descriptive detailing off in its own little box. At worst, you'll have a passage in which half the sentences are crammed with stuff happening, half are filled to the gunnels with descriptive detail, and none of it is remotely functional as narrative because that Actuality-Exposition Structure turns it all into leaden deposition.
Remember: prose is about communication but narrative prose is about conjuring; if it doesn't conjure, it ain't narrative.