Lots of coke as somebody said last time it was asked :)
Now, seriously, I don't know. For Bo Rhap he wrote all the notes in a phonebook, but it doesn't mean he did it with all the songs
Something notable on Freddie is that he had a thing for counterpoints between the bass voice and "the rest". Note in Bo Rhap Opera section or Somebody To Love, the bottomline follows a different "curve" than the rest, which are parallel between them.
That's not only related to voices. In his orchestral arrangements Freddie used to put the double bass in a kind of counterpoint with the rest (violins, violas, cello). Note the amazing double bass parts he wrote in 'Was It All Worth It', 'Innuendo', 'A Winter's Tale', 'Don't Try So Hard'...