You can do this writing exercise from your own point of view or the point of view of a fictional character, depending on your mood or what sort of writing you prefer to do.
Set a timer for 20 minutes. Set pen to paper and describe something you or your character was not meant to see or overhear–a private moment, a confession, a deep trauma, a raw emotion or exchange. Write from that raw place, that knowledge of possessing something you shouldn’t and being unable to take it back or change it. Don’t worry about punctuation, grammar, or the rules of writing. Just write feelings.