The whole truth. Nothing but the truth. So help you God.
The simple answer is, a lot of hell would break loose.
I guess part of being a friend is agreeing to lie, or at least tactfully omit a little bit of the truth. I mean, I think about the people I interact with on a daily basis. What would I say to them if I were telling the total truth?
You're a lech.
You're an emotional infant.
I'm sick of you making me look bad around the department.
You're the most self-centered, self-important person I've ever met and I'd love to burst your bubble.
I hate you for being better at my job than I am.
You're a user and I can't believe nobody's called you on it.
I love you. Not romantically or sexually or even in a friendly way, I just love you like I love the sunrise in the morning and the leaves when they're changing.
Fucking grow up already.
And who would I tell these things to? Some of my best friends. Would I? Not if I could help it. (And just because you read my journal doesn't mean you should assume one of these is directed at you. Most of the people I'm thinking of don't even have LiveJournals.) Just because you like someone, love someone doesn't mean you don't recognize that they're flawed. And you may even tell them when they're flawed. They may need to hear it.
But don't be surprised when they tell your flaws back.