Mimesis is essential to contemporary narrative forms including literature, I commonly identify it as being manifest on two levels, the first regards how certain events are relayed, consider the two examples below.
|Jake pulled out his 45 automatic and fired two shots, Emile fell to the floor like a sack of spuds
|Jake's heart pounded so fiercely, he could feel his pulse throbbing as he clasped the handle of his 45 automatic. The impulse of the recoil from his two shots jolted the frame of his weapon violently and he felt the knurled handle rasp the heel of his palm as the noise of his shots sang in his head. He heard a stifled mew and then a crump, as if a sack spuds had fallen from shoulder height, when the smoke cleared he saw Emile's body laying motionless on the floor.|
The first example is self explanatory but I think we can see it falls into the, once upon a time technique of relaying events. The second one doesn't describe the events as explicitly, events are implied rather than recounted but it includes more detail in an effort to evoke an image inside a reader's head, in other words to mimic that scene in the reader's imagination, mimesis!
The second form that mimesis manifests itself in narrative, is way that a the broader elements of narrative, that is: plot, unfold or the method used to reveal them to a reader or audience. This is where, show don't tell comes in. If you have a character that undergoes some change, be it a positive epiphany or some negative metamorphosis of a virtuous person into a malicious one, you don't leave it with someone uttering: "Oh yeah, Darth used to be a nice guy then he fell into a volcano". No what you do is, you show that story, because as human beings, what we're told means almost nothing, what we learn means everything.
Right, is that clear? probably not I imagine, so I'm going to include a clip from a rather excellent animated tv show that epitomises this concept rather succinctly, not least because, show don't tell is its central theme. Or rather link to that clip because it's one where embedding has been disabled.