Opening the book…
The fastest way to make an invented culture feel false is to make everyone in it the same: all the warriors proud, all the merchants greedy, all the mountain folk dour. Real societies seethe with disagreement, faction, heresy, and generational revolt, and a fictional one that does not feels less like a people than a mascot. Internal variety is also where stories live, because conflict within a culture is often richer than conflict between them. Beyond craft, the single-note culture carries a moral hazard, sliding easily into caricature, especially when a fantasy people is a thin recoloring of a real one; giving a culture its own arguments is both truer and kinder.
For every culture, invent its internal fault lines: the reformers and the traditionalists, the devout and the cynical, the region that resents the capital, the young who find the old ways absurd. Give it dissenters, hypocrites, and exceptions, and let your point-of-view characters hold minority opinions within their own people. Show the same custom meaning different things to different members, embraced by one, endured by another, mocked by a third. When you notice yourself writing a culture whose members all want and believe the same thing, break it, because a people worth visiting argues with itself.
Scale and focus set limits; a people glimpsed for a single scene may reasonably be sketched in broad strokes, and not every walk-on can contain multitudes. Some stories use a deliberately unified culture, a cult, a hive, a regimented state, as their subject. The danger is the unexamined monolith, especially one that flattens a whole people into a single trait.