The things your character says are dialogue. The things she thinks are inner dialogue. If inner dialogue isn’t part of your fiction toolbox, you need to find out why inner dialogue matters and when to use it. The why is fairly easy.
Inner dialogue is a great way to tell your reader how your character feels about something. Becca couldn’t believe she had wrecked her brother’s brand new car. He had worked for months to make the downpayment. All that work gone in a matter of minutes. Yet again she had let him down and this time she’d done it in a big way.
But the key to making it work is to dwell on her more important thoughts. In yoga, a mind that wanders is called a monkey mind. Jon, in my class, says I have a barrel of monkeys mind. He is not wrong. In a matter of seconds I can note the sound of rain, wonder if we have any more of the good coffee at home, and remember that I need to put the laundry in the dryer. But really? Who wants to know about the inane chatter of my monkeys? No one.
Instead, stick with things that matter to the story and that your reader can find out only by being told. This can include what the character wants (goals), why she wants it (motivation), and the stakes. Use the inner dialogue to give your reader important information about the character. Don’t just tally a higher word count. Make it matter!
For a really good list of types of information you can share through character thoughts, see Mary Kole’s Writing Character Thoughts. This is something I’ve been giving a lot of thought because my critique group is always asking me “How does your character feel about this?” Clearly, I need to make sure that the inner dialogue I include is meaningful and not just chatter.