ISFJ has the "Behind the Scenes" interaction style while the INFJ has the "Chart the Course" interaction style. So when working on a task the INFJ will prefer to make their own plan first and then guide others according to the plan or just work on it themselves. An ISFJ is more likely to look for feedback from other people ahead of time and be inclusive and build a consensus before moving forward.
Also from my (limited) experiences so far with the two types ISFJ's tend to be more even tempered, while INFJ's are much more sensitive especially to other people's emotional states. The ISFJ can read other people's emotional states as well, but it doesn't "get inside of them" the same way that it does an INFJ.
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At first my ISFJ friend and I look really alike, after all we are both very well-mannered, reserved, and interested in helping others. But talk to us and it becomes really obvious that we are not that alike at all. I tend to discuss mainly abstract topics pertaining to the psyche, future happenings that may or may not actually occur, and say things like "This is what he/she might have been thinking." My friend tends to discuss fond memories (which I do as well, but not nearly as much) and says things like "This is what happened." If someone brings up a topic that is purely speculative and has no direct effect on her existence she will become bored and will try to hide a "who cares?" face. While I'll be very interested, just because.
I don't know if that made any sense. Oh yeah, that is another difference. ISFJs will make sense