Interplay is the way two or more things act on each other, back and forth, like a kind of dance. You hear it about light and shadow, about music and pictures, about two people in a talk, about two ideas in a book. The noun is the usual form: ‘the interplay between A and B’. There is also a verb, to interplay, but it is less common and sounds a bit like a book. Though actually, this ‘play’ is not a game with a ball. It is closer to ‘play’ in ‘the wind plays on the water’ — movement and effect, not sport. It is a grown-up, essay-style word; in easy talk people often just say how two things affect each other.