The We Can't Stop hitmaker was so tired after walking the red carpet with the country music star, she began dozing in her seat in the theatre.
Miley tells Bbc radio DJ Nick Grimshaw, "I had to go to the premiere and then I slept once I got in. I walked the carpet and I made my dad come with me and I literally laid down. He's like, 'You cannot do this at the theatre right now! You're at the premiere, the actors are sitting right there.' I'm like, 'I can't do it, I'm exhausted! I don't know what's going on because I haven't seen the other ones'. I've only seen the first one."
The 20 year old goes on to reveal her mother, Tish Cyrus, stopped her from watching the Potter movies as a youngster, adding, "My mom wouldn't let me watch it when I was a kid. She said it was witchcraft, she said it was dark. She didn't know what I was gonna turn out to be! My mom had high hopes. She thought Harry Potter was gonna be the worst!"