Counsel for the Prosecution, Sir Hugh Beady-Scrimshaw QC (hereinafter referred to as The Prosecution):
Sooo...Mr. Flatulent...in your statement to the police, you claim to have seen your wife, Mrs. Edith Flatulent, enter your house at 4:33 precisely, thereby proving that she could not have been at the High Street Jewellers at the same time conducting a robbery with a sawn-off shotgun.
Mr. Clement Flatulent, 33 Bullshit Place, Dorking, Surrey (hereinafter referred to as The Witness):
That's right, guv'nor. I was watching telly at the time, see? And the Generation Game was on an' I 'ave a clear view into the hall from my settee. She just came in the door, went into the front room, and left a few minutes later. But, like I said to the rozzers, if she was 'ere, she couldn't 'ave been there as well, innit?
The Prosecution: (with a theatrical flourish pulls a drawing from the voluminous folds of his gown, and acknowledges applause from the gallery with a graceful inclination of his head) I present to the court a plan of Mr. Flatulent's house...
...From this plan, it is quite plain that if Mr. Flatulent was indeed sitting on his settee as he claims, then he had no sight of the front door, nor of the door to the front room. I put it to you, Mr. Flatulent, that you did not see who entered your house and cannot be sure that it was your wife.
The Witness: Oh 'eck.
Mrs. Edith Flatulent: (sobbing) It's a fair cop.
Substitute "The Author" for "The Witness", and "The Reader" for "The Prosecution", and you have a situation all too easy to get caught in. I know I often get carried away writing scenes and making assumptions about who can see what, only to have to come back and rewrite parts where I realise that what I've put on the page drives a bulldozer through the laws of physics and spatial consistency.
Anywhere that line of sight becomes important, I find it essential to draw a picture. Plans and diagrams are invaluable tools in the writer's armoury for dealing with spatial consistency.
The most common tools here are simple top-view maps and plans to establish the spatial configuration of a setting. I use floor plans of buildings and ships, and maps of various scales for towns, regions, and whole planets.
Less obvious uses are to establish realistic travel times between A and B, whether from one room of a mansion to another, or one town to the next.
Of importance in action scenes is to confirm that the action you are describing has room to take place. Does anyone remember cartoons like Tom & Jerry, where chases took place down endless hallways that would require a house the size of Buckingham Palace to accommodate? You don't want your treasured WIP to end up like that, do you?
Finally, don't neglect the third dimension, particularly for line of sight questions. Can you really see from the bedroom window into the neighbour's yard? If in doubt, draw a vertical section to scale to see if you have line of sight over that eight-foot-high wall.
Or what about seeing from the top of that hill into the valley below? Anyone remember drawing contour sections in geography class? Your poetic description of the car chase the hero watches from the trig point might be all for naught if you haven't confirmed your line of sight. Often all you can see from the top of a hill are the tops of other hills!
Of course, you could just stick with what you've written. After all, nobody will notice.