These are gross generalizations, but are some of the trends that I’ve observed. For the INFPs, I don’t know for sure because I’m not inside their heads, but this is how it looks to me:
INFJs are more likely to care about getting their message out or have an agenda for where they want a conversation to go or what they have to say. INFPs are more open to listening first without processing it all right away.
INFJs have an internal structure for measuring everything against. INFPs are have very passionately held feelings about some things, but are less likely to have strong feelings immediately about whether something is good or bad.
INFJs need several go arounds before conflict is completely resolved. Kind of like having a bunch of junk that needs to have places found for it. Nearly always there are a couple of stray items that are found later. INFPs are likely to take a final stance on something and expect it to be done with.
INFJs tend to be more directive when giving instructions while INFPs tend to describe the situation and expect the hearer will infer what needs to be done.
INFJs are more likely to try to influence other’s behaviour, particularly if they care about them and it is destructive. INFPs are more likely to let them learn for themselves and not want to influence the process as much.
INFJs are curious to know how people work and ask questions to flesh out their understanding of a person so they can better prepare for how to deal with them. INFPs tend to take things in as they come without trying to direct the interaction or plug the new details into a preexisting structure in their minds. They are more likely to filter things through the lens of their own personal experiences.
INFPs have strongly held views, but they are based on a personal criteria more than one decided by external factors. Therefore they can be quite unconventional, compared to INFJs.
INFPs have a goofier and very imaginative sense of humour and are more likely to have more gears in between serious and joking modes of communication.
INFPs are often less practical in every day matters and sometimes have a tendancy to avoid dealing with problems. INFJs do that when they are overwhelmed by details, but generally like to face problems more directly.
INFPs are less emotive and appear dispassionate compared to INFJs. However, they are much more passionate under the surface in some regards. INFJs are more emotive, and given the right circumstances passionate express it. While they have layers, they are likely to want someone to understand everything about them inside out. INFPs seem to like some privacy and operate more internally. INFJs need to discuss things with people to organize them in their own minds, while INFPs take in information but do not need people’s input the same way to decide how they feel about something.
Not always, but INFPs tend to be the stereotypical emo kids at school who also like anime. INFJs would be the ones who are friends with a variety of fringe groups, but are not part of them themselves. They’re more anthropologists.
There’s a bunch of others, but these are the ones that come to mind right now.