"Informal interview" after an initial screening phone call?

I was contacted by phone and asked a few questions about my availability (this is a casual/relieving role), national police screenings (a requirement of this job), my current role as she saw on the resume, and she basically explained the job in some more detail and asked if this would be something I am still interested in.

I said yes, and she invited me to an informal interview as she'd like to meet me in person and have a chat. I thought normally there's a formal interview before any additional ones. Is this unusual? A new procedure? What should I expect? How should I prepare?

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