How did you interpret them, if you interpreted them to be significant?
In my opinion, I feel like they were part of Diane's creation that was 'Betty'; I sort of imagined her to be this all american girl who was close with her grandparents, their pride and joy etc.
I thought them screaming at her at the end and driving her to suicide sort of represented how she was being haunted by her past and also by the fantasy she had created in her mind.
Aren't there a few versions of the couple throughout the film?
Like the drunk couple that 'guide' Rita to Havenhurst....and Camilla and Adam at the dinner party that 'guide' Diane to the hitman.
I thought they were extraterrestrials who are keeping Betty/Diane/Rita/Camille in their zoo.
shareAs nothing in the film is literal, they're a representation of all the Midwestern morals that she thought she left behind. They are her feelings of guilt and disappointment in herself personified, all the "shoulds" rattling around in her head, which is why they were approving on the trip out and eumenidies come to claim their due at the very end.
They're one of the more straightforward things in the movie.
My interpretation is that she met the old couple when she first arrived to Hollywood and had a similar encounter. After all the failure and humiliation she went through that moment haunts her. And she replays that interaction in her dreams.
shareThe old couple are Camilla and Adam. There's a scene inside the limo when they're looking at each other that looks exactly like the way Camilla and Adam gazed at each other at the dinner party after they announce the engagement.
I can't take credit for this explanation; the site, Club Silencio, caught onto the similarities between the two sets of characters: http://www.mulholland-drive.net/theories/18.htm
The Old Couple represents her conscience. They are a composite of all the people who believed in Diane as she tried to make it in Hollywood.
shareThere are seeming layers of referents and motifs throughout the film. Much of what transpires in MD can be traced to classic mythology. And in this case, as an example, Irene and her companion may represent the Erinyes (the Furies; the Erinyes are from the Underworld and avenge homicides).
Certainly this fits with your interpretation and perhaps offers an even deeper insight.
My very first thought when I saw that scene was that the old couple were just some people Betty had met on the plane. On long plane flights you often become friendly with a fellow passenger or two. They showed up at the end of the movie because Irene had told her, "I'll be watching for you!"
All the theories about them being Diane's "psyche" or whatever doesn't seem to match the rest of her dream in which all the characters in her dream were based on REAL PEOPLE. So it's safe to assume the old couple did exist and did meet Diane in real life at some point.
The moment when we saw the old couple grinning strangely in a car seems to confirm that they were just briefly acquainted with Diane. When you meet someone briefly, you only have a limited impression of them. Maybe the only impression Diane had of them was that of a happy smiling couple. If you dream about people like James Dean or Marilyn Monroe you always picture them as they were young and glamorous, because you never saw them any other way.