What's a more primal human fear than falling? It keeps us from doing stupid stuff. Heck, even babies know better than to crawl off a cliff.

Hitchcock knew that fear and used it in a number of his films. (Source)

In NXNW, the falling motif is everywhere:

• Vandamm plans to toss Eve out of a plane.
• Evil henchmen Valerian and Leonard fall off the cliff at Mount Rushmore to their doom.
• Eve dangles in terror off the same cliff for what seems like forever until Thornhill rescues her.

For that last one, we see her face in closer and closer shots as we're sure she's about to fall, and oh no no no—

Suddenly, she's pulled up to safety—but she's now not on the cliff at all. The film sneakily cuts to Thornhill pulling her up into the berth in their train compartment for a little matrimonial activity. It's an abrupt and genius move to give the audience a 180-degree emotional turnaround.

Hitchcock wouldn't have let her fall anyway. In an interview with the AFI, he said that you can't put the audience through horrible suspense without giving them some relief. They'll get angry with you, or else give themselves some relief by starting to laugh; either way, the scene's a failure. (Source)