Hi Nesrine
From my own experience, one thing to watch out for (if it can be discerned from their published work at all) is whether they have a strong angle on any of the areas where your work overlaps with theirs. For example, my external was selected on the basis of having used similar methods to mine, but it turned out in the viva that she's now highly critical of these methods - something I couldn't have known. If there is anything in their writing that indicates a particular stance, though, I'd say it's worth paying attention to that.
Good luck, and try to get some relaxation before your viva if possible - I think that as long as you have a good handle on your own work, that is the main thing you can do in terms of preparation.