Fans in attendance and those on Twitter and Facebook were also given the chance to quiz the panel and host Steve Jones on the singing competition, which will award a record-breaking $5 million recording contract to its winner.
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Emma Patterson attended the Q&A for Digital Spy to find out what the judges had to say when quizzed by fans about the talent and changing contestants' lives. Here are some of the highlights:
Simon, how did the US auditions compare to the ones you've done in the UK?
"They felt tougher this time. I definitely recognised the talent was on a different level over here and the audience definitely responded differently too. There was a huge connection between the performers and the audience members that I've not really witnessed before now."
Do the auditions give you a long enough time to really assess someone?
Simon: "You need to blame the producers, not us [if people don't think they get enough time]! I'm all for everyone working longer hours! (laughs)"
Paula: "The thing you need to understand is, these auditions are the chance of a lifetime, so if you really believe in yourself and you really have the talent, then you have to go out there prepared and be ready. Five minutes is enough if you've done that."
Nicole: "Five minutes is actually a long time and I think you know in seconds whether or not somebody has the X Factor."
Did you see people with potential in all the categories this year?
Simon: "I see a genuine winner in every category, I always do. This show is to prepare people for their success and stardom outside the competition - that's what this show is designed for."
Are there any of the auditions that didn't make the show but we can see on the internet?
Simon: "Yes, a lot of the interviews and auditions that didn't make the final edit are indeed available on the official X Factor website, so go on there and check them out."
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What does it feel like to change somebody's life through X Factor?
Nicole: "It's an overwhelming feeling and it's why we get up each day."
Steve: "Regardless of talent, if you've got the guts to get up on stage in front of these good people (the judges), then I've got your back. Whether you get it or you don't!"
What's been the most difficult thing about the show so far?
Simon: "When attractive girls turned up to audition, they all got labelled 'DOA', which stands for 'dead on arrival' as coined by Nicole and Paula. So LA Reid and I had to fight to keep those girls on the show!"
Nicole: "So not true, so not true!"
You've got four extremely talented people judging this year, so how do you come together to decide what your collected X Factor is?
LA Reid: "We vote!"
Simon: "And if they're good, it's often a no-brainer."
Paula, who is going to be doing the choreography this year?
"There's a creative advisor who will be helping me but I will be doing a lot because I like what I do and I'm really good at what I do!"
How long do you think it'll take for Steve Jones to be voted Sexiest Man Alive?
Nicole: "I told Steve, dude you're going to get so much [attention] once this show goes on the air! (laughs)"
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X Factor's world premiere was slightly delayed in the beginning, with Cowell jokingly telling the audience that they should blame the producers because they had been editing the programme until the last minute.
"This is a very exciting night for all of us. This has been a long time coming and we've had a great time making the show," he said. "The show is all about finding a star and I think tonight you'll see we've done well and found some great talent - and I'll apologise in advance for some of the people you're also going to see!"
He also joked: "Paula we were going to put subtitles on tonight but the tape was late so if you don't understand, I'll explain later what she was trying to say!"
The winner will land a record-breaking $5 million recording contract prize and the chance to follow in the footsteps of Britney Spears and Michael Jackson by appearing in a Pepsi Super Bowl commercial next year.
X Factor was open to all individual singers and groups from the age of 12 upwards, and record-breaking audiences turned out for the auditions over the summer.
The X Factor premieres Wednesday, September 21 at 8/7c on Fox and continues Thursday at 8/7c on Fox
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Watch the new X Factor USA promo video below:
Browse a gallery of photos from the X Factor screening below: