He could hardly stand. He sang the high notes anyway.
In 1990, during the sessions for Innuendo, John Deacon and Roger Taylor were playing a chord loop. Brian May heard a circular figure in it and took the idea away. He and Freddie Mercury agreed the subject. Mercury was too low on energy to write the whole lyric, so they spent one afternoon on the first verse, a clown whose makeup hides the pain, before Mercury left for treatment. May finished the second verse later, and a bridge that moves like Pachelbel’s Canon. He thought the title might be too corny. Mercury told him to keep it.
Queen and David Richards produced the album at Mountain Studios in Montreux and at Metropolis in London. May played guitar and a Korg M1, and sang most of the backing vocals. Richards suggested the key change into the second verse. Deacon played bass. Taylor played drums. May’s guide vocal sat so high he could only reach the top in falsetto. The next time Mercury came to the studio he could hardly stand. In earlier years he had shouted at May for writing too high and ruining his voice. This time he did not argue. He drank a couple of neat vodkas, propped himself against the desk, and sang the song across three or four takes.
Innuendo came out in Britain on 4 February 1991. The Show Must Go On is the last track. Parlophone released it as a single on 14 October 1991, aimed at Greatest Hits II. The British B-side was Keep Yourself Alive. It peaked at number 16. There is no new film of Mercury singing it. Rudi Dolezal and Hannes Rossacher, the duo DoRo, cut a montage of Queen videos from 1981 to 1991. Mercury confirmed in public that he had AIDS on 23 November 1991. He died the next day. The single returned to the chart and reached number 16 again.
I said, “Fred, I don’t know if this is going to be possible to sing.” And he went, “I’ll fucking do it, darling.” Vodka down, and he went in and killed it, completely lacerated that vocal.
(Brian May, Rolling Stone, 2010)

The first time the surviving band played the song in front of a crowd was the tribute concert at Wembley on 20 April 1992, with Elton John singing and Tony Iommi on rhythm guitar. Deacon’s last public performance with Queen, in Paris in 1997 with Elton John and the Béjart Ballet, included it. That version is on Greatest Hits III.
The vocal on the record is the 1990 take. A man who could not stay on his feet for long, a couple of vodkas, three or four passes, and the note May had been afraid to ask for. The video is other years of the band. The song is this afternoon in the studio.