I had to attend an interview for my first UK passport. As Beatlemania said, it wasn't anything to stress about - just a collection of super easy questions to confirm your identity. Think questions like: How do you spell your last name? When did you move to the UK? What's your mother's name? What's your partner's name? What's your address? Do you work in the UK? Who is your employer?
Nothing to trick you out, just questions to confirm you are who you say you are.