How to identify a specific 707 variant? I recently saw a very interesting documentary film made in 1980 about a narrowly-averted airline disaster in which the flight crew were incapacitated and a retired pilot, with the help of other crew members and experienced ground controllers, managed to land the plane safely. 

The aircraft in the film is a 707, though I was puzzled by some of the interior shots, in which I thought it looked much more like a DC-8 (it's been a very long time since I flew in either). The footage of the cockpit was actually quite baffling, and I really didn't recognise it at all.

What exterior and interior features would help identify the particular 707 variant?

The documentary is apparently quite famous, so perhaps someone even knows the tail number. It's simply called Airplane! - very informative, and I learned a great deal about the aviation - highly recommended.
Don't know about the interior shots but for the exterior:


  In-flight shots use a model, some ground shots use N6723, c/n 18989/492, a TWA 707-131B.


This is from the awesome website IMPDb Internet Movie Plane database