How Grey's Anatomy's Active Shooter Episode Affected Cast Members

The two-part Season 6 finale of "Grey's Anatomy" is an emotional rollercoaster for fans, as they witness grieving widower Gary Clark (Michael O'Neill) take revenge on Dr. Derek Shepherd (Patrick Dempsey) and the other surgeons who couldn't save his wife. For the "Grey's Anatomy" cast, including Chandra Wilson, Sarah Drew, and O'Neill, the finale was as emotional to film as it is to watch.

In an interview with Entertainment Weekly, Chandra Wilson revealed that she cried for two-and-a-half hours after wrapping up Miranda Bailey's iconic elevator scene. Determined to get wounded surgical resident Charles Percy (Robert Baker) to an OR, Miranda is devastated to learn that the power is off and the elevators are inoperable.

Despite the tears that came while filming these episodes, Wilson was thrilled to be part of such intense scenes, including hiding under patient Mary Portman's (Mandy Moore) bed and watching Gary's feet pass by. Wilson said, "I just loved the terror of that and wanting the audience to have a good time with the fact that Bailey was scared to death and Mandy Moore above, she couldn't say anything even though she was crying underneath the sheets. It was great."

Sarah Drew had panic attacks while filming the Season 6 finale

For Chandra Wilson's "Grey's Anatomy" co-star Sarah Drew, she didn't find the experience of filming the two-part Season 6 finale to be so great. In the tell-all book "How to Save a Life: The Inside Story of Grey's Anatomy" by Lynette Rice, Drew revealed that it was rather traumatizing. In the finale, her character, April Kepner, is the one to find her deceased friend Reed Adamson (Nora Zehetner) in a medical supply closet after slipping on her blood.

"Putting yourself into that kind of panic mode does something to your body because your body doesn't know that this is not happening," said Drew. "I had nightmares and panic attacks while we were shooting that, even for several days after we finished. It was very intense and scary and hard to go to those places and then leave them at the door..."

Once home, Drew often had to actively remind herself that she wasn't in danger and that she didn't actually see a friend and colleague murdered.

Michael O'Neill needed therapy after portraying Gary Clark

Playing the role of a homicidal shooter understandably negatively impacted Michael O'Neill, who told EW that he went to therapy afterward. Though he's grateful that Season 6, Episode 23 ("Sanctuary"), and Episode 24 ("Death and All His Friends") are some of the most watched and discussed episodes of "Grey's Anatomy," this isn't the type of job he'd be able to take on again. "I had a producer ask me not long ago if I would do the same role now, and I said, 'No,'" recalled O'Neill. "I don't think I can do it now. I'm older. Life looks different now."

In fact, when writer Shonda Rhimes called him personally to offer him the part of Gary Clark, O'Neill almost didn't accept. He told Yahoo! Entertainment, "I was so afraid of that being sensationalized, and I didn't want that in the world." However, once O'Neill got to understand more about the character's human side and Gary's deep love for his late wife, he agreed. "It was important to show the fracture that brought this man to that desperate action."