First, let's contrast ニヤニヤ (niyaniya) with its close cousin, ニコニコ (nikoniko). Both describe a smiling expression, but the emotional shades are quite different. Nikoniko means “smiling; friendly-smiling.” Think of someone genuinely pleased by good news, or a friendly shop assistant. The smile is open and welcoming, showing sincere pleasure or affability. You would choose nikoniko if you want to convey a sense of happiness, ease, or amiable warmth.

Niyaniya, on the other hand, describes a grin or smirk. It suggests a knowing, often self-satisfied amusement. It can be a little mischievous, implying the person is enjoying a secret, or finding something funny that others don't get. It's less about warmth and more about a private, almost smug, satisfaction. You would use niyaniya when describing a character who is amused in a way that’s slightly underhanded or superior.

Consider these scenarios:

Scenario 1: A young detective has just solved a difficult case. His superior officer approaches, congratulating him heartily. The detective responds with a nikoniko smile. The scene highlights the detective's pleasure in a job well done. The superior's pleasure is genuine.

Scenario 2: The same detective, earlier in his investigations, has just found a vital clue the other investigators missed. He hides his satisfaction, nodding slightly. As he turns his head, the detective gives a niyaniya grin that crosses his face. The audience understands he's pleased with himself and now knows something others do not. This time, his joy is private, even a bit arrogant.

Another related term is クスクス (kusukusu), or “chuckle.” Kusukusu implies stifled laughter, usually over something mildly amusing. Consider a group of friends sharing an inside joke. Kusukusu wouldn't work for the detective in Scenario 2. The detective's emotion is more pronounced, more deliberately directed.

Finally, we have エヘヘ (ehehe), a soft, possibly self-deprecating giggle. A character might use ehehe if they are embarrassed. This makes a sharp contrast with niyaniya. Niyaniya would be completely wrong here; it suggests a character completely at ease with his own inner self, in a way the other words would not.

In short: use nikoniko for open, friendly smiles; kusukusu for chuckling; ehehe for nervous giggling. Use niyaniya for that private grin that signals amusement, a hidden advantage, or mischievous satisfaction.