Avoid putting them in a way of emotional/social obligations(Fe), by all means necessary. So in that respect, yes spontaneity is a good thing. Though again, Fe is a left-brained function. For this reason INTPs dont do well with emotional improvisation. Avoid putting them on EFPish emotional roller coasters. Emoting gradually will be important and easy for you to do as an INFJ.
Honesty and clarity are paramount. Make sure you make it obvious that you're honest, direct communication would be very helpful. Clarity is important because INTPs have an intense need for logical order, that in combination with their FJish approach to personal matters may make them quite allergic to ambivalent and ambiguous behavior in person-centered situations.
Give a lot of space. Allow for them to initiate personal discussions, yet keep yourself as open as possible. You're just going to have to be patient with their slowly elevating Extroverted Feeling. This will be taxing on your resources, yet almost certainly, allowing for them to be as private and slow to emote will be profusely rewarded in the long run.
Once more, autonomy also tends to be central to their values. Make extra sure that you keep your J from becoming imposing. Keep in mind that whatever decision they will make, must be guided only by their thinking and assessed on impersonal criteria. Suggesting that this be done otherwise will necessarily be met with stubborn resistance.
If they get on your nerves, dont be afraid to confront them, even if you be tactless about, this is unlikely to damage your relationship. They are unlikely to take that personally, and on the contrary will have more confidence in your honesty as this will foster direct communication, as aforementioned tends to be of very high value to INTPs.