Whether it goes well or goes badly, it will be one of the most intense relationships of your life - the one you have with your doctoral supervisor. Love them or loathe them they can't help but be a big part of your doctoral experiences, whether that's from being too hands on and demanding or too distant and disinterested. So, the question has to be asked - can you be friends with your supervisor?
I would say that I have been friends with about 50% of the doctoral students I have supervised or am supervising. Some were my friends before they became students, some were students who sort of morphed into friends over the process and just kind of stayed on in my life once the PhD journey was complete. This doesn't mean I hate the other 50% who were/are perfectly charming people, it just means my relationship with them is not as deep as with my friends. The really important thing though is to have a conversation emphasising the difference between between Caz-who-is-my-friend and Prof-Haigh-who-is-my-supervisor because it needs to be clear that there is a difference; without that demarkation it can be difficult to maintain a supervisory objectivity when assessing the quality of a students work or assimilating a supervisor's comments.
And therein lies the problem, the PhD experience can be so complex and so intimate that what is professional academic interest and esteem towards each other can easily be misconstrued into something that it isn't. Recently Lee Yarwood-Ross and I did some research on students opinions of their supervisors. One student in an online forum asked "Is it normal for a supervisor to insist upon candlelit supervision sessions?" Er...NO!! That's NOT friendship, it's not even marginally appropriate academic behaviour and to any student in that situation my advice would be 'RUN. RUN AWAY AS FAST AS YOU CAN!' I would offer much the same advice to the supervisor whose student was perplexed because she had told him she was in love with him and was upset to find that now he was reluctant to see her alone any more.
That, of course, is why having more than one supervisor, friendly or otherwise, is a condition to be devoutly wished !