Introduction:
In the story "Evans Tries an O-Level" by Colin Dexter, Evans, a prisoner, drapes a blanket around his shoulder in order to conceal his efforts of changing dress to look like McCleery. This action plays a crucial role in the plot and adds to the suspense and intrigue of the story.

Explanation:
Evans, a convicted criminal, is serving his sentence in a high-security prison. He devises a plan to escape from the prison by impersonating a Scottish warder named McCleery. To execute his plan, he needs to change his appearance and clothing to resemble McCleery as closely as possible.

Concealing his efforts:
Evans realizes that he needs to change his prison uniform into McCleery's clothes without raising suspicion. Therefore, he cleverly drapes a blanket around his shoulder. By doing so, he effectively hides his actions and conceals the fact that he is changing his clothes. The blanket acts as a makeshift curtain, shielding him from the prying eyes of the prison guards and other inmates.

Creating a diversion:
Additionally, draping a blanket around his shoulder not only helps Evans conceal his efforts but also creates a diversion. The prison guards and other inmates are more likely to be focused on the unusual sight of Evans wearing a blanket rather than paying attention to his actual actions. This diversionary tactic gives Evans the advantage of being able to change his clothes without attracting unnecessary attention.

Enhancing the suspense:
The author, Colin Dexter, uses this action to build suspense and keep the readers guessing about Evans' true intentions. The description of Evans draping a blanket around his shoulder adds a layer of mystery to the story and leaves the readers wondering what he is planning and how he will accomplish his escape.

In conclusion:
Evans drapes a blanket around his shoulder to conceal his efforts of changing dress to look like McCleery and to create a diversion. This action adds to the suspense of the story and helps Evans execute his plan of escaping from the prison.