It is how they approach it. INFP's are very passive with this initally. If they don't like something or don't want to do something, the respond: "well... I don't know... I/We could just do this... How about we do this instead?... That's nicer/better..." It is rather passive agressive because they don't want to upset others, but their internal values come first above all else. There is alot of uncertainty in the tone of voice (even though interally there isn't much uncertainty), and there are alot of pauses. The INFP doesn't want to send out any kind of signal at all that they are stepping on someone elses toes. They do this by being as passive as possible.When an INFP is crossed enough, they will become exacting, and for lack of a better word, merciless, and they will use you as much as possible.
INFJ's are also passive, but are not as passive. If they don't like something or don't want to do something, they respond: "I'm not so certain that will work out. You know, there is a better way to do it after all. Why don't we try this out, and see where it goes. If not, we can look into something else. How does that sound?". INFJ's will often leave a sort of "open ended" question at the end of it, to give the other person a sense and feeling that they have a voice and say in it. INFJ's know very much what they want, but they also take into concideration what other people want. As such, they will bend themselves around options to try to please everyone. They also do this in presentation of their ideas. They sound certain with things, speak clearly, and ask questions. There will be pauses as well, but these arent passive pauses like the INFP's, they are more "I'm thinking about this, how do I say it" pauses. If an INFJ is crossed enough, they will just have total disregard for everyone else, do exactly what they want, and if someone get's in the way, oh well.