For instance, in a MS. I pick up from the pile on my table I read:
"He paused when he reached the drawing-room door and glared at her, livid with rage. She returned his look with one of haughty indifference. Then he left the house, and as he walked along the cheerless streets, he clenched his fists and hissed between his teeth, 'You shall suffer for this.' She, meanwhile, rang the bell for tea and resumed the novel upon which she had been engaged when he arrived."
Told in the third person, it is easy to let the reader know what he and she were thinking and saying and doing at the same moment. But supposing you were writing all this in the first person with yourself as the heroine, it would not be so easy to convey the same information to the reader. You could write:
"He paused when he reached the drawing-door and glared at me, livid with rage. I returned his look with one of haughty indifference. Then he left the house, and I rang the bell for tea and resumed the novel upon which I had been engaged when he arrived."
But if you wished to let the reader know how the bad-tempered creature clenched and hissed, you would have to get at it by some round-about means—your dearest friend might call at the moment and tell you that she had just passed him in the cheerless street clenching and hissing; or some other such device could be employed. But all this involves extra thought and care in the construction of the story.
A Stumbling-block to the Amateur
Amateurs are much given to story-writing in the first person; it seems such an easy method (when they know nothing about it); they invariably see themselves in a leading part, and make the hero or heroine do and be all they themselves would like to do and be. But they never go far before they trip up against this block of stumbling—the impossibility of the first person singular "I" being in two places at the same time, and seeing inside people's hearts and brains, to say nothing of their locked cupboards and secret drawers.
Also, the beginner is apt to forget the rôle he is supposed to be playing when he puts himself into a story, and he lapses, at intervals, into the third person.
Sometimes, in order to dodge the difficulties, an author will write one part in the form of a diary, thus enabling a character to talk about herself (it is usually a feminine character who keeps a diary!). Then, when the limitations of the first person singular hamper the progress of the story, the diary is dropped for a time, while the author revels in the all-embracing freedom of writing in the third person.
This is a weak method, however, and plainly a subterfuge; being practically an announcement that the author could not or would not take the trouble to work the story through in correct form. It is also bad from an artistic standpoint; it does not hang together well; past and present tenses are apt to get mixed; it produces an unsatisfactory feeling in the mind of the reader, who so often is in doubt as to whether the author is writing as a character in the story or merely as the author—and anything that leaves a confused, unsatisfactory feeling in the reader's mind is poor art.