At the end of Season 1, how did Addison already know about Meredith sleeping with Derek?
Richard called her up and told her. That’s why she came out. Richard brought her out to Seattle because he didn’t like the goings-on between Derek and Meredith. And not because it was an attending and an intern. He didn’t like it because he likes Addison and he feels a responsibility to look after Meredith.