Drew explained that Andrew Perkins (James Tupper), the hospital's trauma counsellor, will have to approve every member of staff before they can return to work.
"It's going to surprise you," Drew told Zap2It. "People who you'd think would be eager to get back to work aren't going to want to go back to surgery.
"The hospital's still running in the background. It didn't shut down at all. Some people will get back to work quickly - for others it will take longer. There are a few people who try to fake their way through counselling to get back to work, even though they're not ready."
Drew also promised that the events between the finale and the new season's premiere will be explored.
"There will be flashbacks to just days after the shooting," she said. "You'll see where everyone is and how they got there. You know, the trauma affects different people in different ways. Everyone is handling things at a different pace - everyone has different coping mechanisms."
Grey's Anatomy returns on September 23 on ABC.
> James Tupper teases 'Grey's Anatomy' role