The interview is obviously concerned with your project, your PhD and your suitability for both. But it's not just about what you bring into the room.
Unless you're applying for an advertised PhD or a place within a more defined research programme, you also need to make a case for the fit between your project and the university (or department) that's interviewing you.
There's a simple way to do this. Talk about them in the interview.
It's easy for Arts and Humanities students to feel like they don't actually need much from their university. Access to the library and its inter-loans service. A supervisor to email and meet with at regular intervals. An email account. Maybe a desk somewhere.
The reality, though, is that you and your project represent a commitment on the part of the university. Part of this is financial (the time and resources you need cost money, after all) but just as important is the investment a university is making in you as a representative of their research work - and a participant in it.
All this means is that you need to demonstrate that you and your project are a good fit for this university. That involves knowing something about its researchers, their interests and their past and present projects.
It won't hurt to also mention any resources or facilities you plan to take advantage of. These cost money and suitable postgraduate researchers are a great way to justify that investment.
Does the library have relevant archival resources, or a subscription to important digital repositories? Is there a departmental research group you'd like to contribute to?
Finally, spare a thought for 'impact'. Universities are very keen to reach out and engage with the general public - and doing so effectively can be a big driver for funding.
Could your project offer opportunities for collaboration with the local community around your university? Is there something you can offer to existing public engagement and outreach activities within your department?