Question

1. Look at her facial expression.
2. Look at her appearance.
3. Look at her looks.

4. How does she look?
- She looks satisfied?

5. How do you think she looks?
- I think she looks unsatisfied.

(Can we use all the expressions? Do we have to use a singular expression or a plural expression in #2 and #3 ?)

Answer 1

Plural in 3, but singular everywhere else.

1. facial expression = the look on her face

2 and 3. appearance, looks = could be just her facial expression, but more likely this is referring to all of her, such as these:
Her appearance is very stylish.
Her looks are quite ordinary.
She looks disheveled.
http://www.answers.com/topic/disheveled