What makes the guitar on Dire Straits' "Romeo and Juliet" sound like that?

I had wondered about that for years.  The song I'm talking about is the great Dire Straits song on arguably their best album, Making Movies.  The guitar at the beginning of the song sounds unusual.  That's because it's a resonator guitar, a modified guitar invented in the 1930s in an effort to make the traditional acoustic guitar louder.  I know that seems odd to those of us who were born after the dawn of rock, but remember, in the 1930s the invention of the electric guitar was still many years in the future.
Now I know what that unusual-looking guitar is on the cover of Dire Straits' later album, Brothers in Arms.  It's a resonator guitar, specifically a National Resonator.  
All of this is simply intro to the point that during the relatively short life of the band, Mark Knopfler (and his brother David on the albums Dire Straits and Communique) did a lot of great guitar playing.  It's a subject I'll come back to, because I am still listening to those records 30+ years later after I first heard them ( and I think I got most of them almost immediately after they were released).