I freeze in interviews, tips? - 8 weeks in

I get tongue-tied when I am asked about my weaknesses. I have practised answers but they sound stiff. How do you prepare answers?

Prepare three stories with a clear beginning, action and result, and adapt them to the questions.

Thanks for writing this out. I went through something close to it 7 months ago, and it settled down once I kept a simple log of every call and form.