Preparation is key. Remember that you are the best subject expert on yourself. Expect questions about the agency, their recent work and the advertising industry at large.
If you are interviewing for a creative position, you will be asked to bring examples of your work so be prepared to talk about your role in the work you show and the process you used.
Thoroughly read through the brief for the role and make a list of the responsibilities and skills that are required for the role. Come up with examples of times you have displayed those skills or held those responsibilities and practise your responses.
Thoroughly research the agency and search any news stories about them. Get a feel for their achievements; client wins, latest advertising and recent hires. Check them out on LinkedIn and follow them so that you are current with their news.
Find out who will be interviewing you and research them on LinkedIn and in other spaces. See what they write about and what their interests are.
This is not an exhaustive list but practising responses to these will have you interview ready. Be clear, concise, and confident in your replies.