"I invented a little scenario: John's gone away on holiday, and he's just rung us up and says, 'Just finish the track for us, will you? I'm sending the cassette, I trust you.'
"That was the key thing — 'I trust you, just do your stuff on it.'
"I told this to the other guys, and Ringo was particularly pleased and said, 'Ah, that's great.'
"It was very nice, and it was very reverent toward John.
"There were a lot of strange goings-on in the studio — noises that shouldn't have been there and equipment doing all manner of weird things — there was just an overall feeling that John was around.
"We put one of those spoof backwards recordings on the end of the single (Free as a Bird) for a laugh; to give all those Beatles nuts something to do. I think it was the line of a George Formby song. Then we were listening to the finished single in the studio one night, and it gets to the end, and it goes, 'zzzwrk nggggwaaahhh jooohn lennnnnon qwwwrk.' I swear to God. We were like, 'It's John. He likes it!'"
—- Paul McCartney
The cover to the Free as a Bird single features artwork by John Lennon; The cover to Real Love is a retro-photograph of the Beatles with a slight
Free as a Bird was intended to be the opening track to Beatles Anthology Volume 1, Real Love was intended to be the opening track to Beatles Anthology Volume 2, and Now and Then was originally intended to be the opening track to Beatles Anthology Volume 3. It was instead used as the closing track to the 2023 remastered Blue Album.
"Think about me every now and then, old friend."
— Last words of John Lennon to Paul McCartney
Although Free as a Bird and Real Love were released in 1995, the third reunion track, Now and Then, was put on hold for eighteen years.
The three remaing Beatles worked on all three tracks in the mid nineties, but George Harrison vetoed completing the third track due to the extremely poor quality of the John Lennon demo recording. He felt that it wasn't of a sufficient quality to be able to use it in a song.
But Paul McCartney never forgot it. During the time from 1995 to 2022, he wanted to complete and release the song. He spent 18 years mulling over the tune, writing and rewriting, composing, until the technology reached the point where Lennon's original demo recording could be cleaned up and restored to a crystal clear state and the vocal and piano tracks separated from the rough mono recording.
So this song has literally been in the making for 35 years.
More details in the link below:
THE UNTOLD STORY BEHIND THE LAST BEATLES SONG
... and in the documentary: