<P> For the show's scheduling, the live episodes are usually arranged to take place over the course of a week, with the semi-finals aired mainly on weekdays, and the final aired on the weekend; in some series, the schedule featured a break between the live episodes of the semi-finals, due to coverage of live events . All live episodes are divided into two parts when they are aired - the first half focuses on performances, each beginning with a short clip of the semi-finalist / finalist's background, and ending with comments by the judges; the second half occurs after voting is closed, usually after another programme has aired between the two parts, and focuses on the results of the phone vote, with a guest performance taking place prior to the announcement of the results . This format was initially used for the live finals only, up until the start of the semi-finals in the fourth series . </P> <P> For the first four series after the show began in June 2007, the judging panel consisted of music executive and television producer Simon Cowell, television and West End star Amanda Holden, and newspaper editor and journalist Piers Morgan . In 2009, the producers made plans to alter the show's format to allow for a fourth judge when the third series was set to begin, with plans for Kelly Brook to be the new judge on the panel . Less than a week after the series began, the producers dropped this change on the belief that this alteration to the show's format would complicate it, resulting in Brooks being credited as a guest judge for that series . In 2010, Cowell fell ill during filming of the fourth series and was unable to attend the Birmingham auditions, leading to Louis Walsh stepping in as a guest judge in his place, until he had recovered . </P> <P> In 2011, the panel saw its first major change, when Morgan revealed he was leaving the show to travel to America and begin filming of his new show . As Cowell had announced he wouldn't be present for the fifth series' auditions, due to his busy schedule with launching The X Factor USA, the show's producers decided to amend the panel, and recruited comedian Michael McIntyre, and actor, singer and former America's Got Talent judge David Hasselhoff, to join Holden during the auditions, with Walsh returning as a guest judge for the London auditions due to Hasselhoff's commitments with a pantomime at that time . When the series entered its live episodes, Cowell returned to oversee the acts as a fourth judge . Later that year, in October 2011, both Hasselhoff and McIntrye declined to return for the sixth series, while Cowell announced he was returning full - time to the show . </P> <P> On 2 January 2012, the producers revealed its decision to adopt the use of a fourth judge for the programme's format, announcing that both Cowell and Holden would now be joined by David Walliams and Alesha Dixon for the sixth series, with the latter moving to the talent show after deciding to leave BBC's Strictly Come Dancing . During filming in February, Holden was unable to attend the London auditions, due to having given birth to her daughter at that time which had led to her suffering some after - effects from her pregnancy . As a result, the producers brought in actress and model Carmen Electra as a guest judge until she recovered . In subsequent series, the line - up remained as Cowell, Holden, Walliams and Dixon, with no further incidents except for the eighth series in 2014 - Cowell missed the first day of the Manchester auditions, leading to Ant & Dec filling in for him, and was also absent for the final day of the London auditions - and the tenth series in 2016 - Cowell was late for an audition, and was temporarily replaced by Walliam's mother Kathleen, who was attending it . </P>

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