Adding to what machzero said:
With regards to personalities they're looking for I can't really answer the specifics of it but you'll notice it when you're there. Those of us who weren't low recommended seemed similar in many ways but different in others, same as quite a few of the BFTS guys I spoke to, so while machzero is right there is a mould to some degree there is some play for differences.
The two weeks does allow them a pretty good insight into who you are, especially under stress so it's hard to fake who you are to meet some idea of what they're looking for, you'll revert to type at some point and it will be picked up. Obviously you have to take with a grain of salt whatever anyone says after their result but one lad did come out and mentioned something we'd all picked up on was brought up by the board. Given the amount of time in the group interviews and the board they'd definitely managed to pick it up well before that. So like they said make sure you are yourself, they're going to find it anyway. There really isn't much to do once you're there, you've either done what you need to for personal development or you've not, try not to worry about that just take the time to enjoy it.
It took me 3 years but I finally got a recommendation.