The 'Heart Attack' singer admits that her 'X Factor' USA boss - who was once dubbed TV's Mr. Nasty - is really nice behind the scenes and she has a huge amount of respect for the British music mogul despite having disagreements with him on the show.
The 20-year-old star told Company magazine: ''He's a grump. An old fart. But he has a heart that nobody gets to see. If he walked into this room, he'd treat everybody the same - he shakes everyone's hand.''
Demi, who will take on her second season as a judge on the talent show this summer, previously claimed that she might have taken on the role too soon after checking out of rehab.
She said: ''I don't think I was ready when I started on the 'The X Factor'. In an ideal situation I would have maybe waited a year or more. I dived into work really quickly and maybe that wasn't the best idea at the time, but the place I am in today is so much better than I could have ever imagined, so I'm really glad.''
Demi also revealed that checking in to a treatment facility to help her overcome her struggles with bulimia and self-harming in 2010 was a life-changing move, especially because it proved who actually cared about her.
She said: ''A couple of months before I went to rehab, I had a birthday party, and there were a couple of hundred people there. It was full of people who I considered my closest friends.
''When I turned my phone on after being in rehab for three months, I expected lots of text messages and phone calls. I had four texts. That was a wake-up call.''