Team Matching on unrelated area

How will I show experience in this field to the manager?

You can't right? You mentioned you have never worked in this area?

Will the behavior interview questions focus on the specific area of interest as well?

I recommend being upfront that you don't have experience in this area (but are eager to learn). Otherwise you will be wasting time fielding these questions you can't answer.

Is just showing interest in the area enough or expertise is required? This is senior/staff role.

It depends on how picking the hiring manager is. AI ethics & fairness is a team only a big company will have, so I assume you're talking with something in the Big Tech range. At these companies they know that fundamentals are the most important and hope that you can learn on the job. You should play up any experience you have where you had to adapt to a new domain quickly.