I've composed an "INFP or INFJ?" write-up as an anecdote to that awful website of a similar name & to the P/J behavioral stereotypes that confuse many.... I wanted to focus more on thinking differences as opposed to behavioral ones, since MBTI is really about the effect of cognition on personality. Any behaviors mentioned here are for illustrative purposes only, to highlight a thought pattern, not to describe the type literally.
INFPs & INFJs, please let me know what you think. I obviously feel more qualified to describe INFPs, but I also don't want to inject too much of, well, ME into it. Oh, and if some of it seems like fat, then let me know what I can cut to simplify it.
Things to keep in mind:
The dominant function is the main focus of the mindset.
The auxiliary function supports the aims of the dominant, aiding a person in self-growth.
The tertiary is a relief-function, used when under stress to "escape", but can lead o unhealthy thinking patterns.
The inferior function is an uncomfortable, unnatural thinking that triggers use of the aux function to fall back into a healthy thought pattern.
To explain that last bit....using INFPs as examples:
Lenore Thompson's function ordering has a person preferring functions in the same brain hemisphere, which means an INFP would have a better handle on Ti than Te, because it is introverted judging and a right-brained form of thinking. I know many are thinking, wait, isn't Te a natural balance for Fi? Yes, it still would be in this theory. Here's how: Te is the most inferior thought process for INFPs, as its the true opposite of Fi, and using it sort of triggers Fi-doms to turn to their far better developed extroverted function out of frustration - that function being Ne, the auxiliary function for an INFP. So in a sense, Te does work in tandem with Fi, as its opposite, not so much providing balance as triggering a healthy FiNe thought pattern. This makes its role more pivotal for an INFP than Ti, which at most will support Fi valuations with a logical "backup" line of reasoning (comfortable, but not very accurate much of the time). Te usage means growth becomes it kicks Ne into gear, so to speak. Of course, this is all my opinion based on other opinions ....
Onto the descriptions.
Fi in the dominant position - evaluates to determine significance/meaning based on an internal measure. This internal measure is an imagined ideal. Basically, the Fi-dom hones these concepts of the ideal by imagining scenarios & using their response to them to gauge their significance. The ideals formed are basically fundamental concepts of the human condition & what is good/necessary in relation to it. Fi reasoning is focused on perfecting concepts of what is harmonious, ethical, aesthetically valuable, perfect, etc. This means feeling is turned inward, and even emotion is internalized as a part of a process of refining what is ideal. There is little interest to affect others with their feelings. Attempts to relate feelings to the world are done by perceiving....in the case of the INFP, that's Ne.
Ne in the aux position - perceives how Fi ideals relate to real world & vice versa, but in terms of general concepts, not specific, concrete ways (that's more Se). It also helps Fi ideals become refined by seeing new possibilities, or what I call "imagination fodder"; the core concepts of what is ideal become less specific as the Fi-dom imagines more ways they can be met in reality. So Ne takes Fi ideals & presents numerous possible ways to fulfill them (which is great, because Fi is damn picky). The more ways one sees to fulfill a value/ideal, the more open, flexible, and novelty-seeking they may become. This is reflected in INFPs' demeanor. They don't want to be boxed into some rigid structure or they might miss a possible way to fulfill an ideal. They seem on an endless quest for finding some semblance of this ideal in reality, and that quest requires flexibility and spontaneity to pursue ideas as they emerge.
The tertiary function is a stress-relief function, which means for INFPs, Si use may be a matter of preferring the familiar as an emotional comfort (when otherwise they'd prefer novelty), and focusing on detailed, sensory, almost reflexive hobbies to relax (such as creating detailed drawings or playing an instrument). They got lost in comforting, familiar, sensory activity or information reviewing, basically. This can go bad when they get stuck on reviewing negative past experience. Since Si is very idiosyncratic in nature, it can add a level of eccentricity also, the very opposite of the "earthy, grounded SJ" when appearing in an INP.
Inferior Te in an INFP can mean trying to make objective, logical sense of something when they hit a road block, then becoming frustrated as that road block & line of reasoning tears down their Fi ideals, and so they chuck it & turn to Ne to find an innovative solution to get around the road block & meet their Fi ideals. Te combined with stress-relief Si can mean making something (often related to a Fi ideal) more efficient by organizing or categorizing it based on a routine method, down to the nitty gritty details, and of course, criticizing other people's non-standard methods (this is when we clean our rooms! ). This is the perfectionism of Fi gone ugly, when we become demanding & critical. However, you can see how it can lead back around to Ne brainstorming - how can I do this better, what other options are there, etc. Then, a positive thought pattern is restored.
Ni in the dom position - In order to grasp it, grasp this first: Si forms impressions of the tangible, literal world & focuses on those subjective impressions over the literal objects, whereas Se focuses on the literal objects themselves. With that in mind, Ni forms subjective impressions of existing concepts and focuses on these impressions over the actual objective concepts, which is what Ne focuses on - the concepts behind what is real & how these connect & what they could lead to. So Ni perceptions are sort of the individual perception of the background of the background of the world, honing on the core concept of many concepts by focusing on their impressions of these concepts (these impressions can amount to imagined connections which highlight the core concept). It's why partially why Ni seems more like a confluence & Ne like tree branches in their perception forming. It is also why it seems mystical, abstract and generally removed from reality. It often produces someone more focused, composed, zeroing in a goal or idea & using structure to accomplish it.
Fe in the aux position - evaluates Ni internal perceptions according to an external measure to choose which perception is most accurate or valuable, and then those perceptions are used to see how to accomplish a Fe objective. As mentioned above, this creates a more "composed" effect in the individual; things are narrowed down instead of branched out. The external measure mentioned is real world significance & what things mean in relation to a consensus. What produces harmony between people, & between them & the environment, and what is a common standard we can use to ease communication/interaction? Fe reasoning is based on these measures, using externally visible results, and it spurs one to create structure of some kind to maintain these good results (ie. harmonious relations via cultural standards of politeness). Fe reasons according to a scale that is set according to common needs of some group that person identifies with (and that group can simply be "human"). The perceptions of the INFJ are often used to see how to create these structures, which is why they are more likely to go against the norm than, say, an ISFJ, because their perceptions are focused on concepts over literal impressions. They may see a better, new way to meet the same Fe objective by discarding the common view & honing it on its core meaning.
Stress-relief Ti for INFJs can mean focusing on concepts of what is logical in order to ease being overwhelmed emotionally or pursuing leisure activities which involve some kind of strategy. They "detach" basically.
Inferior Se in INFJs can mean having the literal world, as it is, thrust on them, and being disturbed when this conflicts with their perceptions of what the world is conceptually, as the literal meaning feels confining & stagnant. Then they turn to Fe to manipulate these literal aspects to something more closely resembling their perceptions (ie. influence others to further a concept of what reality must become, so the world is no longer static with one literal meaning). This Se use combined with Ti stress relief can also produce an interest in physical activities that involve some kind of logical framework/strategy that they can manipulate (see how symbolic that becomes? ). This is when INFJs become evil masterminds or con-artists. However, once again, you see how Fe thinking can restore the healthy thought pattern by personalizing situations & the people in them instead of just viewing people as abstract pawns in a strategic game to fulfill a vision. What is good, important & meaningful as a common ground between people is used as the deciding factor.