It largely comes from a TV special (I believe written by Alan Jay Lerner, but don't hold me to that) which included what was supposedly a re-creation of rehearsals for My Fair Lady. It's pretty clearly dramatized, but great fun. In part, seeing Rex Harrison chewing up the scenery as he rants and raves through practicing "I'm an Ordinary Man." But far better than even that -- most especially for those who still haven't gotten over the fact that Julie Andrews didn't get to put her legendary performance down on film -- we see Andrews ostensibly training to learn how to do a cockney accent and then testing it out with an absolutely terrific performance of "Just You Wait."
But here's the thing: that isn't the real treat in this video.
The person who put this video together edited onto the end a scene from an Ed Sullivan Show tribute to Lerner & Lowe. The composers famously used that broadcast to heavily promote Camelot, which had recently opened and was only doing fair business at the box office. Instead of using the air time to do retrospectives of their past work, they filled up a lot of broadcast with the cast of Camelot, performing almost 20 minutes of the show, in order to showcase the new musical. The next morning, there were lines around the box office.
But still, the broadcast was a tribute to Lerner & Lowe. And since they had the one of the stars of My Fair Lady already there for Camelot, they had Julie Andrews re-create one of her numbers from My Fair Lady. And so, thanks to all that, we finally get to see Julie Andrews, in costume, as Eliza Doolittle, with members of the original cast performing "Wouldn't It Be Loverly."
And her performance is so freaking wonderful that your heart also breaks while watching this and seeing hat could have been. What should have been. And you're left with two pieces of gratitude: Thank goodness that this footage exists. And thank goodness for Walt Disney, If you're going to have sloppy seconds, how much better can it get than Mary Poppins.
The video ends just a few seconds before the end of the song. But it doesn't matter. It's just loverly.