T. R. Knight will not be a regular this season, while former recurring cast member Jessica Capshaw has been promoted to a series regular. Martha Plimpton played a role in a multi-episode arc, that started with the season premiere episode. This season was the first to showcase episodes focused on different characters. While most episodes followed the same ensemble principle that were present in the first five years, certain episode focused on one character in particular such as Derek ("Give Peace A Chance"), Arizona ("Invest In Love"), Owen ("Suicide Is Painless") and Alex ("Sympathy For The Parents")

The sixth season introduced several new residents joining the Seattle Grace Hospital staff because of the merger with Mercy West Hospital. Those residents include Nora Zehetner's Dr. Reed Adamson, Jesse Williams' Dr. Jackson Avery, Robert Baker's Dr. Charles Percy and Sarah Drew's Dr. April Kepner, who was fired in episode six and rehired in episode thirteen.

Kim Raver joined the cast on November 12 as Dr. Teddy Altman, a cardiothoracic surgeon and Iraqi-war veteran who served with Dr. Owen Hunt. It was announced on January 4, 2010 that Raver has become a series regular.

Izzie Stevens will no longer return to Seattle Grace Mercy West Hospital, as Katherine Heigl has been released from her contract.

Source: Wikipedia