Remember: no matter what point of view you write from, you MUST stick to what your narrator knows. (Your readers can figure things out before your narrator does, but you can't reveal information your narrator isn't supposed to know while they're narrating.)
Here's an example:
My sister scrunched up her face, trying not to say what she's thinking.
If your book is in first-person POV, your narrator doesn't necessarily know what other characters think or try to hide. They may if they are deeply familiar with the other person, but even so, you'd have to disclose how your narrator reached those conclusions.
This type of issue often manifests in overusing the word "because."
Example:
He said nothing to me because he was too sad.
Instead, show the sadness nonverbally and let your readers see it for themselves.
OR, you can omit the "because" and split the sentence, so you're not explaining away others' actions.