Social energy
One question: does time with people add to your reserves or spend them? Not whether you like people, not whether you are good with them, and not whether you are nervous — those are separate things that share a vocabulary. Someone who finds company restorative can still be awkward; someone who finds it costly can be warm, funny and excellent at it, and still need the drive home in silence.
The cost is real in both directions. A week of enforced quiet does to one person what a week of back-to-back company does to another. More on social energy.
Emotional style
Think of it as amplitude: not whether you have feelings, but how loudly they arrive and how long the echo lasts. Two people read the same curt email; one thinks someone is having a bad morning and moves on, the other is still turning it over at dinner.
A high reading is a finely tuned alert system — early, sensitive, and prone to firing on things that turn out to be nothing. That is a cost and a genuine advantage, usually in the same week. More on emotional style.
Why these two
Gather a large number of descriptions of how people differ, look for the structure underneath, and two dimensions keep separating out first and largest: one about engagement with the outside world, one about emotional amplitude. This has been found repeatedly, by different people, with different item sets, in different decades and different languages. In the five-factor framework they are extraversion and neuroticism.
Cross them and you get four regions — which map closely onto the four classical temperaments, either a remarkable coincidence or evidence that people have been noticing the same two things for a very long time. We did not invent the axes. We picked the two with the best evidence behind them and built the instrument and the advice around them.
The four regions
The names are landmarks, not boxes. Most people sit nearer the middle than any label suggests, and the interesting thing about anyone is usually where they sit between two of these rather than squarely inside one.