And as the training progressed, I was able to inspect and adapt both her ability to sit and stay (until released) and the level of distraction within which she had to maintain those stays. As I've mentioned, we practiced at home and in fun and distracting outdoor spaces. I also increased the challenge at home by going to the door and knocking on it; sometimes the knocking would be accompanied by door rattling or knob shaking - anything to make the sit/stay process that much harder for a happy, excitable dog.
When she showed that she was ready, I introduced the further challenge of an actual visitor.
Now, I'm sure that my dog would tell us, if she could, that one of the toughest challenges she had to face in her young life was to keep her rear end on the floor in the presence of her favorite person in the world: her dog walker. But she did do it. Again, at first it was for a mere few seconds; over time she was able to hold her stay until released.*
*Of course, I should also say that this required the cooperation of our amazingly patient dog walker (we miss you, Tami!): she would essentially ignore my dog until released and free to say hello – and then she would physically lower herself to greet my pup, so that keeping "four on the floor" was much more feasible.