Stevens shuffles back to the table in front of the room, noisily pulls the chair out, and plops down into it.
This sentence shows imagery and is describing how loud he pulled the chair out. It is unnecessary but helps the reader understand the writing a little more than before and makes it more enjoyable to read.
In the back row a girl scribbles furiously in a fancy journal. Against the far wall a young man in sweat pants and a Minnesota Twins hat snores quietly—his desk empty, except for his head. In the front row Stevens spots Dana, the notorious overachiever, pretending to read Finnegans Wake. Plastering on a smile, he welcomes the students to Intro to Creative Writing
These few sentences show a lot of imagery, not only where everyone is sitting in the classroom, but it also shows how people are acting toward the things they are doing. One example of the would be Dana "plastering on a smile".
Professor Stevens dislikes donuts; the icing gets stuck in his beard. Fridays he breakfasts at Burger King before heading out to the lake, where he smokes cigarettes on the shore and ignores his wife’s phone calls. He idolizes James Dickey. He’s no good at fiction. The Department Chair’s out to get him.
This first paragraph is used to show Professor Stevens is and give us examples of a daily routine he follows every Friday. It gives us imagery and shows us things that are special to him and his persona.