In "Cathedral," what makes the narrator start explaining what he is seeing on television?
It is clear that the narrator initially feels very awkward to have a blind man in his house and also to be watching television with him. Of course, Robert is unable to see, and although the history documentary they are watching does have lots of dialogue and explanation of what is being shown, there are times when it does not, and this makes the narrator feel particularly uncomfortable, because he is able to see what is on the screen, but Robert is not. Note how he describes this sensation:
There were times whe the Englishman who was telling the thing would shut up, would simply let the camera move around over the cathedrals. Or else the camera would tour the countryside, men in fields walking behind oxen. I waited for as long as I could. Then I felt I had to say something.
The narrator only begins to describe what he is seeing on television therefore after a number of silent moments on the documentary they are watching. Even though he does tell Robert what is being shown, it is clear that he did not want to describe what he could see initially, as the phrase "I waited as long as I could" signifies. The narrator at this stage in the story still feels detached and does not want to offer any sign of friendship or openness with this blind person whom he feels is something of an interloper and makes him feel awkward.