How you use Ser and Estar depends on how you understand the reality. For instance, if you have an old friend from school, Peter, and since you know him, he is kind of ugly. Then you don't see him for some time, and you meet again when you finish the university, and he is changed and looking much better than before. Then you will say "Peter está muy guapo". This is because you categorized Peter as an ugly person, but now you are changing him from category. Then you introduce Peter to your friend Anne for the first time. Then your friend will say "Peter es guapo", because Anne is putting him in a category (and not moving from one category to another). This is why you can use Ser o Estar in the same phrases, with just a slight semantic difference.