As someone who naturally freezes when they are put on the spot, what I've found really helpful is to think of the questions they're going to ask, write or type up my answers, and practice sitting and answering the questions out loud a ton of times for each question.
The repetition helps because the first few times are going to be super awkward and it helps to get that out of the way. Then as you get more comfortable you start noticing where you might stumble over something that sounds great written but is harder for you to articulate in real life if you aren't used to speaking that way. Practice changing the words around or using words that feel more natural to you.
By the time the interview happens I remind myself that it's definitely normal and expected to be nervous during an interview (who actually enjoys them?) so just roll with it if it shows. And if I don't get the job, this interview will at the very least be great practice for the next one!