I definitely need a LOT of time to myself to process. I don't talk when I get stressed, I need to be alone to sort through how I feel, and what I think about something. I can't really think about stuff/figure things out while talking. So it's going inwards first, contemplating/processing, connecting the dots or reaching resolution, and THEN I feel comfortable talking about it, just to let it out, or simply share it, or to get feedback or affirmation. But if I think on my own for a while, and really can't figure anything out, I might then talk to others because I recognize I need input, or need a sounding board. But that's only the second step, once I realize I'm getting nowhere on my own. These sorts of things can get complicated -- complicated to know the root cause behind why I go inward, I mean, and don't talk things through with others. I've always been rather solitary, and I think it's just old habit started from long ago when I never talked to anyone about anything personal, and kept it all in. It's only in the past 3-4 yrs that I've really worked on opening up and being more expressive with others, and actually extroverting my inner world.
I don't really know any confirmed INFP's, so I cannot compare myself to them. I am good friends with several INFJ's, though, and I have a lot of similarities with them. It's interesting, though, because although they're all INFJ's, they do have different nuances -- I mean, there's definitely a spectrum, and variance within each type. One of them is so talkative and emotionally expressive that she rather exhausts me sometimes ;-), a few are more talkative than I but definitely have more balance in talking vs. listening, and are more cautious and reserved in expressing emotion, and the male INFJ I know is pretty similar to me, as far as his introvertedness goes. And then a few are what I would consider much stronger 'J's' than I. I've never really felt my 'J' tendencies are terribly strong, but maybe that's just where I'm at now, and maturity. I don't know. Also, I'm just...not as reactive/combustible (for lack of better words) as one, and not as planned as another.
Oh..and I have never really had a life plan. ;-) I don't think that can be tied to J/P. One of the INFJ's I know has always had a life plan, and she's one who has a lot of direction (compared to I). The other INFJ's I know are more floaty, like me, in that way.
But..while several people have confirmed my INFJ-ness and I don't think I'm nearly as mellow, in general, as the INxP's, I do think my Fi is pretty high, and is probably as strong as Fe. So there's that too.