Inner dialogue is an excellent way to give your readers a peek inside the heart and mind of your characters. Readers can’t get this depth of character strictly from the actions you include in your story. You should give them inner thoughts to create 3-D characters with which your readers will fall in love.
The bad news is that there is no hard and fast rule about formatting inner dialogue. Depending on which author, editor, or publisher you talk to, there are as many ways to handle inner dialogue as there are people writing it.