(Four Temperaments Ensemble) How to write a phlegmatic character that isn't boring?

When looking through the examples of Four Temperaments Ensembles, the phlegmatic character is almost always boring and borderline unnecessary. Even just the description of what the phlegmatic character is supposed to be sounds completely antithetical to dramatic storytelling. As Pixar's rules for storytelling say, "Passive/malleable might seem likable to you as you write, but it’s poison to the audience." How do I make this kind of character work?

There are a whole bunch of ways. Phlegmatic personalities are the mirror of choleric ones, in the classical model, and that has quite a few potential interpretations.

Consider a calm, somewhat withdrawn character, prone to ruminating on a situation but rarely taking action, happiest giving advice and letting others do things for them. Maybe they're a bit manipulative, but not in a particularly cruel way; they just nudge people in a particular direction to get what they want, and don't see fibbing or omitting facts to do so especially harmful. This person cares about other people, but they're afraid of having to engage with negative emotions that they can't handle without the possibility of being hurt. They're considerate and kind, but unwilling to put themselves out there or tell hard truths. This person lacks personal ambition and feels ambivalent about the ambitions of others, feeling good for other people when they succeed but disdainful of "hungry" people and utterly contemptuous of those who hurt others to get to their goals—which, on some level, they realise is slightly hypocritical. They don't particularly like themselves, fairly or unfairly, but have no strong inclination to change.

And there you go.