The actress, who plays matriarch Esme Cullen in the Twilight films, reveals castmates met in secret to work tirelessly on their moves for the flash mob, which was created as a wrap gift for the moviemaker.
She explains, "There's about 100 vampires dancing on a soundstage in the snow. We secretly went into a room on our breaks and no one knew anything about it.
"We told the cinematographer, 'Can you shoot this, we're gonna surprise Bill,' and it's (to) the song Sweet Dreams (Are Made of This), and we went out there and just threw down so hard."
Castmate Peter Facinelli was so impressed with the dance sequence he was hoping Condon would be inspired to change the ending of the final Twilight film: "We were hoping that was gonna be the big ending."