Growing up, since we were 6-years-old, the INTP was much more quiet than the INFJ. The INFJ would make observations and express some of his thoughts and opinions on matters, making conversation, while the INTP more often than not simply asked questions and made puzzled faces (as he internally made sense of things that didn't seem to make sense). At young age, the INTP was more drawn to creative and inventive products to learn more about how they came to be, while the INFJ (although creative and inventive as well) was paying more attention to what was going on around us at school. Both of them were (still are) goofballs, but for the INTP it was like he couldn't even help it sometimes and would hold it back some, while the INFJ could enjoy being the group's comic relief and would pronounce himself to act theatrically, interpreting other people's behaviors and voices in a more exaggerated manner for the laughs. The INTP often did things like trying to get from the classroom to the cafeteria by only stepping on shadows, while the INFJ would be rushing to not have to deal with a long waiting line. The INTP would also spend a long time moving on his chair, trying to find the most comfortable sitting position, regardless of what was going on at the moment (even at times when there was a heavy/intense atmosphere, he'd be doing stuff like that). The INFJ was more attentive of how he portrayed himself (and gave more priority to the tone of the atmosphere). The INTP admitted to me that he was irritated by how people often called them "the twins" and not by their names separately. I asked him why that bothered him and he explained it had something to do with how people forget that they are different from each other and rarely looked past who they were together. The INFJ wasn't as bothered about the lack of distinction but could be greatly annoyed when people misread them or put them on a negative light, and it would be more evident on his expressions (while the INTP would laugh and shrug it off most of the time, even though it bothered him too). The INTP was much more absent-minded, the INFJ was more alert though would still get caught in ideas often. The INTP would hear a rumor and break it apart before giving it any validity ((thank you for making the school battlefield more tolerable, INTP buddy)), the INFJ would look at where it's coming from and how it might be possible before discarding it. The INTP inquired the thoughts of others before deciding on a course of social action, the INFJ observed others and made up his mind about what was or wasn't appropriate. The INFJ had to remind the INTP of birthdays and holidays, and/or give him the push to do nice things for special occasions. The INTP would over-think this and come up with incredibly thoughtful stuff, commemorating details that most would have overlooked and shock others with how much attention he was paying while he had seemed distracted. Nowadays they team up for projects. The INTP says he enjoys letting the INFJ sketch out ideas so he can then perfect them afterwards.
