He ran up the steps and knocked on the door. After a few moments, it was opened by a woman with dark curly hair and a strained expression.What you've just read are two simple sentences showing a common situation. So common that the author very probably has not given a thought as to whether her words reflect what really happened.
She wants her character to pay a visit to someone; she wants him to knock on the door; she wants someone to open the door. So that's what she writes.
Sometimes, this is exactly what you need - a fast, simple transition to get you from one scene to the next. But sometimes, by not thinking about what other options you have to show the scene, you lose an opportunity to slip in some revealing information about your characters. Before you go on auto-pilot and write down a few clich