Alan is not as oblivious as David appears to think. He's seen the way he looks at him when he thinks his attention is elsewhere. He's also seen the increasingly dejected air David has whenever Alan appears.
David, clearly, thinks his interest is not returned.
He's wrong. Alan is just not prepared to do anything about it on a film set full of children. The young ones have a tendency to burst in without warning, and Alan won't risk it.
But when end of filming arrives, and the traditional party, the first chance he gets to whisk David off somewhere private… well, *then* he'll show him just how much his interest is returned.