They all have 3 auditions before they go in front of the judges, no-one turns up on the day, it's just not possible. The crowds you see snaking around are the audience for the day, they just insert the "contestants" to interview them. The employees of x factor are walking around with clipboards with the contestants names on with a comment by each one, such as "deluded", "rubbish", "good". I know, I looked over their shoulders, and I was in the crowd. the judges also have these comments available before they see the contestants. So yes, they knew Susan Boyle could sing, whatever they pretended. It's all about what they think makes good TV.