Hiya,
This really depends on what university you are in. I had a few interviews and some were very manageable while others were not so great!
I find that a bit of confidence in your project goes a long way, though I would certainly advise you to understand where possible problems with it might arise and talk about how you will mitigate them. Strangely, i was a cheeky so and so at the interview that I eventually got funding for, but that was because they kept me waiting for over an hour outside the interview room.
Definitely, they will ask you why you want it and what you hope it will achieve and what you want to be doing in ten years.
Also, if i were you, I'd have a good idea of the type of reading you'll be doing!
Hope it helps, good luck.
BoooooooM!