I actually identified with Keirsey's description with INFP at first, because I had a very fantasy-filled childhood (and as an adult, I still do). When I was little, I had stuffed animals to which I assigned voices and personalities and imagined them as living beings (like a Calvin and Hobbes deal).
Although most of my fantasies involved making up movies in my mind and acting them out. Something that concerned my parents, who later sent me to the shrink. I also apparently had a fascination with acting out violent scenes in my "mind movies" (mostly just making gruesome sound effects, though), and so my parents suspended me from watching movies that were rated above PG.
On the other hand, unlike an INFP, I was more interested in the systems I invented in my mind rather than the characters.
When I read Please Understand Me II, I began to identify myself as either an INTP or INTJ, but when I took the Keirsey test on both Advisor Team and Please Understand Me II, I came up as INTJ, along with many other MBTI tests. Overall, though, I identified with the Rational temperament.
Although I am more interested in art than science -- specifically filmmaking, screenwriting, and surrealist art -- but I don't consider myself "down-to-earth" or "realistic" like the SPs, nor do I consider myself a humanitarian like NFs (unless humanitarian is opposite of vegetarian! :P ). I am sort of ritualistic like the SJs, but again, I don't fancy being "down-to-earth" or "realistic."