Kate Walsh made her first appearance as Dr. Addison Montgomery in Season 1’s “Something To Talk About.”
Walsh returned to the series in the Season 8 episode “If/Then,” which took place in an alternate universe in Meredith’s dream.
She became a series regular towards the beginning of Season 2 until the end of Season 3, before she went on to star in Private Practice.
The episode shocked fans with the reveal that Dr. Derek Sheppard (Patrick Dempsey) was married, even though he and then-intern Meredith Grey (Ellen Pompeo) were going out.