The actress, who plays Dr. April Kepner, described the May 7 episode as "huge", "sad" and "intense".
"Everyone bawled their way through the table read for the finale," she told E! Online.
"It's left really unsettled. We don't know what's going to happen to a lot of our characters."
Drew previously hinted that the finale - titled 'Flight' - will be "on par" with the two-part season six finale, 'Sanctuary' / 'Death and All His Friends', which featured a dramatic shooting at Seattle Grace Hospital.
"It will affect everyone, but [this time] it happens outside of the hospital," she explained.
Grey's Anatomy airs on Thursdays at 9/8c on ABC. The eighth season finale airs on May 17 in the US.