Hosts Dermot O'Leary and Caroline Flack were also walking the red carpet as the first filmed auditions began in Liverpool.
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.