19th law:
Let's back up a bit, because the problems with simplistic sun-sign astrology for personality analysis were baked into it from the beginning.
Babylonian astrology was fixed-star and constellation based until around 500 BCE, when they discovered that it was easier to predict eclipses with pre-set 30-degree sectors of the 360 degree zodiac. Eclipses were important omens for them. Signs didn't start out with ordinary people's personality traits: an idea that had little meaning to ancient people. However, constellations and planets picked up the traits of gods and supernatural creatures for whom they were named.
Then the Hellenist Aristotelians got into the act, and divided the signs into Aristotle's 4 elements: earth, air, fire, and water.
So astrology starts out with a lot of ideas that have nothing to do with human personalities. In many ways, personality analysis is a modern concern.
When astrologers wanted to look more closely at ordinary people, the gods' traits and the 4 elements were much of what they had in their repertory to deal with.
A modern astrologer whose books you might enjoy reading is Stephen Arroyo. His book Astrology, Psychology and the Four Elements is still in print. He asked an important question: what is very real for people with a predominant element?
Basically Leo, along with Sagittarius and Aries, is a fire sign. Will and enthusiasm are very real-- and important..
For predominantly earth sign people (Taurus, Virgo, Capricorn) material, practical things are very real.
For air-dominant people (Gemini, Aquarius, Libra) ideas are very real.
To water-dominant people (Pisces, Cancer, Scorpio) feelings are very real.
You can go a little ways with these two principles just knowing someone's sun-sign. However, if someone has one planet (sun) in Leo, and just about everything else in Cancer and Scorpio, water (not fire) is their predominant element. This can make for a moody lion.
Then each sign has a modality or quality: cardinal (Aries, Cancer, Libra, Capricorn.) Fixed (Taurus, Leo, Scorpio, Aquarius) or mutable (Gemini, Virgo, Sagittarius, Pisces.)
Cardinal: initiative-taking
Fixed: persistent
Mutable: flexible, adaptable.
So put 'em together, and we see Leo as the fixed fire sign. Think: "persistent enthusiasm" or "stubborn will." [Hint: key words are very helpful in astrology.]
From Robert Hand, Planets in Youth, p. 33: "If you give a fire child the feeling that adults always use power arbitrarily and without rules, then the child will later use his power in the same way, often against the kinds of people who seemed arbitrary in the past."
Does this ring a bell? I wouldn't describe Leo as vindictive, a toxic quality I associate more with emotionally-driven Scorpio, but someone with a "fixed will" might seem like the elephant who never forgets. He may be acting out of an unstable or even traumatic childhood where authority figures did what they felt like. If so, that's what the young Leo learns to model in a leadership role.
Of course, to use all of this astro-information properly, we need to see the entire composition of the horoscope.
With a dual element emphasis, we might get "enthusiasm for ideas" (fire + air) or a tendency to intellectualize one's feelings (air + water.)
A shortage of one element may lead to a form of compensation (the low-water, air-dominant person who observes others' feelings without the ability to emphasize) and this person will emphasize the predominant element/s, even if it's not the sun-sign.
However, what Robert Hand didn't mention was that if a sun-Leo had a troubled childhood, or became a vindictive adult, you should be able to see this in the chart. For one's own childhood, we look to the 4th house, and then to planets symbolizing Mom and Dad. For vindictiveness I'd look to Scorpio and its rulers Mars (traditional) and Pluto (modern.) Chances are, they're not in the happy pile, if you're seeing squares and oppositions. (With conjunctions, it depends on the planets.)
To really know whom you're dealing with, 19th Law, you'd want to avoid simplistic beginner-level short-cuts. Real human beings are more complex.
Aren't you more complex, yourself?