The Merriam-Webster Dictionary defines literary impressionism as 'the depiction (as in literature) of scene, emotion, or character by details intended to achieve a vividness or effectiveness more by evoking subjective and sensory impressions than by recreating an objective reality.'
So, the literary impressionist is focused on presenting an impression of a character's reality through imagery which incorporates the five senses. We can see examples of this in Stephen Crane's An Episode of War. In the story, Crane never explicitly announces that the lieutenant has been shot, but he paints (in words) the sensory evidence of the soldier's injury. Furthermore, Crane is focused on our protagonist's feelings and sensations in regards to his injury. What the lieutenant sees, hears, feels, and touches encapsulates everything that the injury does to him.
What you and I read is what the lieutenant experiences as it happens. The immediacy of real-time action is evident in works exemplifying literary impressionism. When he is first shot, he is in shock. He doesn't know what to make of it; he 'winced like a man stung, swayed dangerously, and then straightened.' His men are as 'silent, astonished, and awed by this catastrophe' as he is. No one wants to believe that their leader has been injured, much less that he has been shot within the safety of their camp. Everyone tries to act 'normal,' but what is 'normal' supposed to feel like when one has been shot? Let's take a look at how Crane uses literary impressionism to describe the lieutenant's experience as he feels shock and pain in the aftermath of his injury. Bolded words are mine.
He looked at it in a kind of stupefaction, as if he had been endowed with a trident, a scepter, or a spade...This wounded officer engaged in a desperate struggle with the sword and the wobbling scabbard, and during the time of it, he breathed like a wrestler. (We can almost feel the lieutenant's raw desperation in this depiction of his struggle to hold on to his dignity. The subjective or organic imagery alerts us to his emotional paralysis and accompanying physical impotence).
He saw a general on a black horse gazing over the lines of blue infantry at the green woods which veiled his problems. An aide galloped furiously, dragged his horse suddenly to a halt, saluted, and presented a paper. It was, for a wonder, precisely like an historical painting. (This is what the lieutenant sees on his way to the field hospital. Both his injury and the battle scene before him seems surreal; he has sustained injury without benefit of battle. The 'historical painting' imagery alerts us to the lieutenant's shell-shocked state).
To the rear of the general and his staff a group, composed of a bugler, two or three orderlies, and the bearer of the corps standard, all upon maniacal horses, were working like slaves to hold their ground, preserve, their respectful interval, while the shells boomed in the air about them, and caused their chargers to make furious quivering leaps. (Here, kinesthetic imagery provides the impression of movement, violent action, and frantic desperation. Auditory imagery through the booming of the shells alerts us to the noisy specter of war).
A battery, a tumultuous and shining mass, was swirling toward the right.The wild thud of hoofs, the cries of the riders shouting blame and praise, menace and encouragement, and, last the roar of the wheels, the slant of the glistening guns, brought the lieutenant to an intent pause.The battery swept in curves that stirred the heart; it made halts as dramatic as the crash of a wave on the rocks, and when it fled onward, this aggregation of wheels, levers, motors, had a beautiful unity, as if it were a missile. The sound of it was a war-chorus that reached into the depths of man's emotion. (Here again, both auditory and kinesthetic imagery alerts us to the chaotic atmosphere of war on the battlefield).
The above are only a few examples of literary impressionism from the story. Hope this helps!