sometimes (very often, actually) i think about all this "who fell first" thing, applied to zoya and nikolai, and here's my theory (and my headcanon):
i don't think love works that way. there's not a moment in which one of them realises they're in love, like a gesture, a look... nothing like that. it's a journey (and for them, a very slow one). in their case, it's hard to say who fell first, and honestly? i don't think it matters.
what i think actually matters is who realises first, and it has to be nikolai. he's an expert at wearing masks, so he's also an expert at knowing what's behind them. he knows how he changes around zoya and how to conceal his thoughts and feelings on the matter. he uses his masks consciously, but zoya wields her shields as if she had nothing more, nothing beyond. for zoya, her shields are a second nature, but nikolai's are deliberate. while nikolai is fully aware that he's in love with her, she's only just starting to sense it.