The Scottish actor - who plays Owen Hunt in the ABC medical drama - revealed that the new season will include several flashbacks of its characters.
"I can't really say too much because that would be giving stuff away. Flashbacks are featured heavily in episode two and may appear in other ones," he told Entertainment Weekly.
The eighth season concluded with a plane crash that resulted in the death of Lexie Grey (Chyler Leigh), while Mark Sloan (Eric Dane) is also set to depart the series at the beginning of season nine.
Speaking about the characters' reaction to the crash, McKidd said: "The writers have kind of turned everything on its head and under duress and under stress people do act in a way out of character."
On whether Owen cheating on wife Cristina (Sandra Oh) will still be a main issue in the ninth season, he added: "I think we've kind of blown past that in a way because the cataclysmic events of what happened, especially everything with Cristina.
"It's going to come back and affect Owen as well - even though he wasn't on the plane. In retrospect, it's going to come back and really affect him in a big way too and in a way that we don't expect."
Creator Shonda Rhimes recently revealed that the ninth season will feature a Meredith and Cristina stand-alone episode.
Grey's Anatomy's ninth season will premiere on Thursday, September 27 on ABC.