Halliwell appeared at the Liverpool auditions and the veteran popstar made an immediate impact on fans, climbing out of her car window and speaking via a megaphone from the vehicle's roof.
"I loved Geri. She was mental in a good way," said Flack.
"She climbed out of a car window. Who else would do that? Name me someone who would do it."
Louis Walsh also backed the Spice Girls star, denying rumours that they didn't bond on the panel.
"Geri was great fun. I've never said I don't like her," said Walsh. "But there's only four places on the panel and Nicole Scherzinger won."
The X Factor series nine starts on Saturday, August 18 at 8pm on ITV1.
[X Factor Photo Gallery - Judges & Hosts]
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