How to keep a really BIG secret?

EW quotes J.J. Abrams — who created the modern day Star Trek movie franchise (here, here) — on how he keeps all his movie details secret despite such a large scale production. He says:

Yet Abrams says he doesn’t bully and nag his teams on the subject. “It’s not like there are threats, it’s not like we’re begging them every day,” he says. “We just say up front that all the work we’re doing is about making this a special experience for the viewer; let’s preserve that as long as we can.”

I speculate that it’s more about the non-disclosure clause in contracts signed by the employees, contractors, etc. who are working on the project. What is said up front is probably more like a friendly reminder about what’s in the contract and by mentioning it is reinforces the seriousness of the agreement.