I wrote this in another thread...

In an ENTj/ISFj relationship, the ISFj voices his/her concerns about the various options available and their possible outcomes, and the ENTj happily provides an in depth analysis of what would most likely happen in each situation and how things could be handled in each situation if something went wrong. The ENTj addresses the ISFj's concerns about the likelihood of various things going wrong in different situations and then narrows down and simplifies the options. "If we do A, then B will probably happen. C could happen, but it's unlikely because of D, and even if it does happen we can always E. If we do F, there's a good chance that G or H could happen, which wouldn't be so bad, but the problem with that is that we'd also have to deal with I... The question is, do we want to E or to have to deal with I? Chances are we wouldn't even have to E anyways..." When presented with this type of information, the ISFj can either confidently make a decision about what should be done, state what (s)he thinks should be done and leaves the decision up to the ENTj (but wants an answer right away), or asks the ENTj what (s)he thinks they should do and then confidently accepts the ENTj's advice. Generally speaking, the ISFj is the one who is ultimately responsible for seeing that decisions are made one way or another because the ENTj doesn't have the same sense of urgency about finalizing specifics and actually taking the steps required to implement his/her ideas, though (s)he really wishes (s)he did. The ENTj is very appreciative that the ISFj pushes him/her this way.

INTps aren't as willing to put forth this type of effort... when the ISFj tries to ask the INTp what they should do, the INTp says "You decide." In an ESFp/INTp relationship, the ESFp would be more than willing to make decisions and would do so with little hesitation. The INTp would be there to criticize what the ESFp is deciding, and then the ESFp would appreciatively revise his/her decisions based on the INTp's input. Generally speaking, the ESFp is the one who is ultimately responsible for seeing that decisions are made one way or another and pushing plans forward because the INTp doesn't have any sense of urgency about finalizing specifics and actually taking the steps required to implement either of their ideas.

How I've seen this translate into an INTp/ISFj relationship is the ISFj asking the INTp what they should do, and the INTp saying "You decide." The ISFj stresses out and doesn't know what to do and is frustrated that (s)he's not getting all of the input (s)he needs from the INTp in order to make the decision... when it comes to finalizing specifics and actually taking the steps required to implement ideas, the ISFj can't do what (s)he does best because (s)he doesn't feel (s)he has the information necessary to make decisions. After a great deal of stressing out, the ISFj finally makes a decision on his/her own. At this point the INTp casually offers criticism on the ISFj's decision... I'm sure you can imagine how frustrating this is for the ISFj, who is then even more upset that the INTp didn't offer any input or help him/her make that decision in the first place and doesn't wish to go through the whole stressful process again to come to a new decision on his/her own, only to have the INTp criticize it again.
From the INTp's perspective, I could see how the ISFj's demands for more input before the ISFj is even anywhere near making a decision could seem like nagging or excessive worrying since (s)he'd rather his partner come up with these things on his/her own, at which point the INTp could begin to pick the "plan" apart, assuming that his/her partner appreciated his/her input and didn't take the criticism too personally. The INTp appreciates abundant initiative and doesn't mind that there are plenty of flaws that need to be worked out.

Obviously there's more to the relationship than this... activity relations are one of the most favorable relations and the description I just wrote sounds pretty megative. Perhaps one of the INTps who's with or has been with an ISFj can offer input.