Preparation: fairly minimal (lack of time). Although did re-read thesis thoroughly, thought about answers to usual questions like contribution, what would you do differently, any weaknesses, why choose topic etc.
Went in, external said she really enjoyed reading it, and it was a very good thesis. I didn't mention my huge list of typos I'd found.
Some general questions from both about why I chose the topic.
Then about an hour and 20 mins of discussion about my work. Imagine you get stuck in a room with two people who happen to have read your thesis, are genuinely interested, and simply want to know more about your work. That was it exactly. There were a few methodological questions like why did you choose to do x this way and not this way? And a couple of times I had to say 'actually, there is no reason, the other way would have been better come to think of it'. We had a really nice discussion about one result which was unexpected but interesting, and we literally just brainstormed the problem until we came up with a possible answer (which I am now going to test).
The issue of typos came up, and I said I'd found a lot, but I didn't actually hand them my list or anything.
At the end, it was obvious it had gone well, but I had to actually ask what the result was. I said 'so, does this mean it's minor corrections' and the external just said 'oh yes, just correct these typos, it's an excellent thesis'.
Then they asked if I had any questions, and I asked about publications, so we discussed suitable strategies for 20 mins.
It simply could not have been better. I only say this because nobody knows what it's going to be like, it's impossible. And if you search online, you only tend to find the horror stories.