In Jane Eyre, an example of an interior monologue is when Jane tries to decide what to do when she realizes she can’t marry Mr. Rochester because he is already married.
An interior monologue is when a character has a conversation with his or her self. Usually the objective is to work out some issues, or some kind of internal conflict. When Jane’s wedding to Mr. Rochester in interrupted with the terrible news that he is already married to a crazy lady he keeps in the attic, she locks herself in her room. In this conversation she is trying to decide what to do.
The answer she gets from herself is that she must leave Thornfield at once. This is not something she can bear to think of.
“That I am not Edward Rochester's bride is the least part of my woe,” I alleged: “That I have wakened out of most glorious dreams, and found them all void and vain, is a horror I could bear and master; but that I must leave him decidedly, instantly, entirely, is intolerable. I can not do it.” (ch 27)
Jane, whose life has been full of disappointment, has been dealt a terrible blow. She did not want to fall in love with her employer, but he wooed her. He convinced her that he loved her, and she was all set to go outside society’s boundaries and marry above her class. Then she found out that he was married, and not only was he married but his former wife was in the house! The fact that she was crazy and he had to keep her in the attic was irrelevant. She felt betrayed, and had to decide what to do so she could live with herself.