Hosts Dermot O'Leary and Caroline Flack were also walking the red carpet as the first filmed auditions began in Liverpool.
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Halliwell climbed out of the car window when pulling up to the venue and started speaking to the crowds through a megaphone, while stood on the roof in a tiny blue dress.
The former Spice Girl is rumoured to be the favourite to land the fourth judge spot full-time, if she impresses in Liverpool.
Other guest judges expected on the tour include Robbie Williams and Rod Stewart.
X Factor auditions arrive in London next week from May 28-30.