The temperament philosophy is an old, outdated pseudoscience. It's not a very effective method to gauge people's personalities, since individual people are too diverse to be narrowed down into one or two categories. Like the Enneagram, or the Myers-Briggs Test, it tries to take something as complicated as a human personality and narrow it down, or pigeon hole it, into a single category or, at most, one category with few supporting points.
Despite being pseudoscientific and specious at best, they're still fun to work with, because they can sometimes give you a handle on what type of individual you are. You might find that you fit more into one category than the others, or you might find that you fit more into a handful of categories than others. It can give you a label to help define yourself - both a blessing and a curse, because that label you use to help others identify some part of you can become the only thing that defines you, and when that happens, something went wrong. For instance, I'm a very strong Type 3 on the Enneagram, and my MBTI is INFJ (which is a bit of a contradiction, but I score high on Type 1, Type 4, and Type 7, the usual matches for my MBTI).
As an author, however, these can be good starting places when designing characters (I'll admit that I didn't start here. I take parts of my own personality and I break it up and let it grow from there, but that's beside the point; this is clearly intended to be a "do as I say, not as I do" post). I do sometimes take personality tests from the perspective of my characters, just so I can know ahead of time, or help get a better view of who they are as people. It can help you flesh out and define the little people populating your book: it can also help you spot potential conflicts a mile away - and explore it in your book.
This method is how I've been able to flesh out the cast of the Blue Pimpernel in my head, at least, as well as I have. Each girl has a very distinctive personality, with similarities and differences that make them stand out but make it so that it makes sense when they work together, and make even more sense when they rub one another the wrong way and it leads to conflict.
I was glancing through this post on the four temperament philosophy, and it hit me just how easily I can break the cast (or, rather, the main four characters, anyway) into the categories. It helps a lot that, as the author says, "everyone has a little bit from all of them (aka, the P.T. Barnum effect)", but that everyone has one major, defining trait and a minor one. While that may be true for some people, and not for others (it isn't for me), it helps me get a better grip on the characters and come at their personalities from a different perspective.
All samples are from the The Blue Pimpernel: Liquidity. I vouch not for quality, since I haven't even finished the final draft yet, let alone proof read. However, consider this to be a sneak peak of sorts: