Evans tries an O level, A critical analysis of the short story by Colin Dexter

It is clear from a reading of the short story, Evans Tries an O Level, that the protagonist is a shrewd and clever person, someone who understands human nature very well. He knows how to play with effects, perceptions, and the minds of people, especially his jail warders, Jackson and Stephens. When Stephens comes into the cell and he sees a figure on a chair slumped over, with blood pouring out of his head, he jumps to conclusion and assumes that the person in the chair is none other than McLeery! This is apparently how the human brain works, sometimes the visual impact of the scene overrides our rationality and we jump to conclusion without even verifying the facts. 

In the case of Stephens we can say with great confidence that his hasty conjecture, and the visual impact of the man sitting slumped in a chair with blood pouring out of his head was so great that he just could not see the obvious! It is exactly such weaknesses that a criminal targets to make good his escape, or even to commit a crime. But it doesn't end here, it was not just Stephens that Evans targetted, rather it was the Governor, and entire prison machinery that was targetted. Take for example, the deliberately misleading hints left in the paper. The words read, 'You must follow the plan already somethinged. The vital point in time is three minutes...'. The Governor continues to read, '...to the Headington roundabout, where you go straight over and make your way to ... to Neugraben.' and then it hits him like it has been planned by Evans, that the destination was Newbury! The excitement of discovering the secret, the euphoria of having found the answer to a difficult riddle makes the Governor lose his presence of mind! He cannot see that something is wrong and that this might be a red herring.

Evans knew very well that the Governor had  weaknesses, he believed he was very intelligent but was in reality rather 'good for a giggle, gullible governor'! After all, the Governor was no different from his subordinates, Stephens and Jackson. He saw what he was meant to see, his attention was diverted away from what was really happening inside his own prison. He was made to look away so that Evans could make good his escape! Evans was able to exploit the Governor's overconfidence. He was a conman who used his knowledge of human perception to create a virtual reality, a mirage and an illusion. Evans had turned things upside down, he knew that prison officials are trained prevent prison breaks therefore he knew that if he created drama with special effects, then the people running the prison could be fooled into thinking that Evans had made good his escape, not realising that the person sitting in the chair was Evans and not McLeary! Twisted logic, one would term it!
Another feather in the cap for Evans was that the clues were meant to lead the Governor to the very hotel where Evans was lodged. That was part of the devious plan that Evans had for him. Again, the Governor's sense of elation was exploited to the fullest so that when he apprehended Evans in his room in the hotel, the Governor made one more mistake! He let Evans out of his sight, and let him go in a prison van that had already been commandeered by Evans' cronies. The whole escape plan is a brilliant masterpiece of criminal wit. The final escape takes place when Evans is let out of the prison van by his friends, and he makes good is escape, finally!
The moral for prison staff is not to develop any kind of soft corner or sympathy for criminals. Jackson did have a soft corner for Evans. The Governor thought he knew Evans very well. He was perhaps dulled into a sense of satisfaction that he had taken all the steps to secure Evans, although he continued to have nagging doubts that something was wrong. Perhaps, the Governor should have trusted his instincts more than his mind!
The short story is meant to remind us that often seeing is not believing, and that the reality can often be disguised with the help of special effects, like blood pouring out of a wounded man's body. As Madonna says in her song, Frozen,  "You only see what your eyes want to see." It is exactly this vulnerability or weakness that Evans targets in this short story. In the battle of wits between the criminal and his keepers, it is often noticed that the criminal often gains an advantage over his keepers because of his keen observation of their weaknesses. In many cases, this fear of having made mistakes, doubt, and the stress of taking adequate steps to confine the criminal may often rob keepers of the law to see what is not obvious!  It was a matter of great foolishness on the part of the Governor to believe that Evans would erroneously leave clues that would lead to his arrest at the Golden Lion Hotel!