Inner monologue also know as inner dialogue is when a character thinks or talks to himself in the book. Other characters do no hear it. Only the reader gets the insight. This form of communication can either help or hurt a story. It also is great when setting up character development. You learn a lot about someone by what they think.
The Do's and Don'ts (link)
DON'T ramble on without action. Sandwich the inner thoughts with what they are doing or an action. Chop it up. Similar to outer dialogue. I don't want two bobbing heads talking back and forth. I want what they are doing and where they are in there too!
DON'T think too much. A lot of times we have characters who are just thinking too much. That might work with someone who is schizophrenic but normal people act and live. If a gun is at your head, you most likely aren't thinking about much.
DO show DON'T tell. Personally this one is the pain in the ass but necessary. Don't keep telling us that a person is scared. Show us they are. She can't stop her palms from sweating, her eyes keep darting for an exit. Don't just say... She is scared. ******* Star, number, highlight the heck out of this one. Most important. BY FAR!
DO cut the descriptions. Personally I don't really care if you're bed has butterflies on it. I care why it does. Does it have anything to do with her life? If not, cut it. Some description is fine- better good. I want to know where you are but I only want need to know. I'm good with some imagination
DO keep up some mystery. I don't want to know this girls life long story right off the bat. I want to know what I need to to get back into the action. Filter in little things here and there. Don't spill your guts on page one. Trust me, it's a pain in the ass down the road. I had to cut a lot out of my first chapter because I said too much!
I didn't write much but I will elaborate more later. I am a big fan of inner dialogues. I love how it can get the reader to really connect with characters. I also think it's hard to do. I have trouble with it and have had to cut a ton out of my book. You have to really know the character to know what they think. That's very important. Will write tomorrow on examples, and how it affects a story. :)