Following on from the point above, don’t be afraid to throw a few curveballs. Ask questions students wouldn’t expect, bring up seemingly unrelated topics or news stories, challenge their answers, or bring out new material (like an article, literary excerpt or even a Tweet!).
And both with curveballs and more run-of-the-mill questions, make sure you give students a long window to answer. There’s often an instinct to jump in and try to help during a long pause - try to resist!
As you make notes, don’t just assess their answers. Think about their body language, and what they’re communicating to you non-verbally.
Most of all, don’t be afraid to be honest. Of course, you don’t want to knock their confidence too much, but give them proper feedback, offer to coach them, and hold another interview to see if they’re improving!