From TenebrousReflection's link:That really rang true for what I know of my INFP friend. On the surface, we are quite similar and get along wonderfully.Sometimes INFPs deny having ideals or principles since it's possible to equate a reliance on principles with dogmatism and inflexibility. Instead, they prefer to talk about feelings: not momentary emotional states but the personal values on which the feeling function operates.
How we express our emotions is also really different. I *think* I am very emotionally expressive and am always honest as to how I feel about given things. However, after one incident in which I was emotionally overloaded and went into a complete emotional meltdown in front of a group of friends (crying and the works), one commented later that this was the first time she had seen me care/get worked up about something, which absolutely flummoxed me. What I had read as emotionally expressive (most through my words), was not emotionally demonstrative to others.
The INFPs I've known are subtlely emotionally demonstrative. My INFP friend can tend to withdraw when she is reevaluating... not that she would really 'fess up to any conclusions she's drawn.
Don't know if the rest will help, but: I have a lot of hubris in the ideals I hold up and implement, and am willing to draw the line in the sand- and usually am proven wrong with such silly rules/assessments such as "honesty is the best policy" or "everything can be meaninglessly delineated into a 'best answer' multiple choice question". My INFP friend doesn't really share her general ideals, but when someone doesn't measure up, she reacts very quickly. Some guy she goes out with on two dates, but he's not THAT guy (in her mind), he's out. But she's too nice to let him down too hard, and she often goes through more turmoil trying to figure out how not traumatize him than he does after the fact.