I've noticed that the ISFjs I know well really cannot be told what to do by most people. They pretty much do what they think they should do, regardless of whether or not it's what someone else is pressuring them to do. They aren't confrontational about it... more like... strong willed? (And they don't like to be rushed or pressured even if they do intend on doing it?). They may tend avoid unnecessary confrontation (especially with people they don't know well), but not at the expense of doing something they've decided they shouldn't do. They may just ignore the person's requests, or they may say they're not going to do it (as opposed to an INTj I know who almost always says he will and then just doesn't get around to it ), or they may briefly explain why they don't feel they should do it.
Do you think this is a type trait, or more of a coincidence? Keep in mind that you may not notice the ISFjs you know irl doing this unless you're in their "inner circle".
Fi: well defined likes and dislikes, well defined sense of what's right or wrong to them
Se: willpower, strength to resist