Is it normal to talk and smile to myself?

There is someone I know who talks and smiles to herself regularly.

She is a quiet person, and she likes to keep to herself. When she is alone and not occupied with anything to do, I hear and see her talking to herself in a whispering voice. Sometimes, I also see her smiling to herself. She loves to sit in front of the laptop and read the news and other stuff.

After she had a shower, she would sit by herself and start talking and smiling to herself. In that whispering voice. And when I asked who she is talking to, she would lie and give reasons like: I am just memorizing for tomorrow's exam. Or she would say she is not talking to herself, just thinking of something.

Like I said, she only does this when her time is not occupied. Sometimes she would "hide" in the toilet and I could her the whispering sounds and I would know she is talking to herself.

Is this normal? Why does she do that? Constructive and serious answers only. Please don't make a joke out of this.