Still Atahuallpa and Pizarro, The Royal Hunt of the Sun took me to the site of the day:
Internet Broadway Database

The Royal Hunt of the Sun, London, December 1964
The National conquers an empire with its premiere of The Royal Hunt of the Sun, on December 8 1964

Stage directions are not usually the highlights of a script, but it was a peculiarly laconic one that attracted John Dexter to Peter Shaffer's play. It read:

"They cross the Andes."

That line had been one of the reasons the play had been called unstageable by most of London's script readers. Dexter, though, was thrilled by Shaffer's gumption, and by his story - an adventure epic about a band of Spanish conquistadores who conquered the Inca empire, crossing not just the Andes but swamps and plains, too. Shaffer's ambitious script was all the more surprising since he was relatively inexperienced: his first play, Five-Finger Exercise, had had a West End run that was respectable rather than spectacular, and The Royal Hunt of the Sun was his second. Spurred by Dexter's enthusiasm, it would become the first new British play produced at the National Theatre.

Having cast Robert Stephens as Atahuallpa and Colin Blakely as Pizzarro, Dexter rehearsed the cast in two halves so that Europeans would meet Incas on stage almost for the first time. Michael Annals designed a set that would suggest multiple locations and the ceaseless, ravening sweep of history. Its centrepiece was a huge sun, symbolising the Inca empire. During the play the gold would be removed, leaving a dark, glowering hole. Annals collected hundreds of metal bottle tops and hammered them down to make his sun shimmer.

After a tryout at Chichester, the production was ready for London - and the critics were already excited. The Standard's Milton Shulman described it as "trailing success behind it like a fizzing rocket". He praised "the intensity, maturity and intelligence of Shaffer's writing", the "ultra-romantic events" that made up the plot, the play's "extraordinary urgency and relevance" and, most of all "the exotic, imaginative, tempestuous effect of the production", which gave the sun-worshippers' world "an eerie, mysterious glow".

The "exotic... exciting spectacle" also thrilled the Telegraph's WA Darlington, but he had doubts about the writing. He felt that the play's two strands - "a factual, though not realistic, account of Pizzarro's amazing conquest" and "an imaginative study of Pizzaro's own personal search for a faith" - did not always mix. In fact, the performances were "moving in spite of, rather than because of, Mr Shaffer's flow of language". The Guardian's Philip Hope-Wallace felt much the same way, finding "feeble moments" and "less profundity than I had expected".

The Evening News's Felix Barker sounded a rare note of optimism. "In the whole history of pageant drama, has anything more ambitious been attempted than The Royal Hunt of the Sun? Has any other spectacle achieved such visual excitement, and... so touched the historical imagination?" The key to its success was in precisely those impossible stage directions: "Mr Shaffer has not dodged one blood-stained, gold-strewn mile of the fantastic journey."

As for the cast, Barker felt that "no praise is too high". Here, he was not a lone voice. "Colin Blakely," wrote Hope-Wallace, "as the grizzled dying conqueror bears the drama magnificently to its conclusions." Shulman was equally impressed by Stephens, who "breathes warmth and passion into the majestic, transcendent husk of the Inca ruler".

Despite all this, when the production transferred to Broadway, it was recast, with David Carradine as the Inca king and Christopher Plummer as Pizzarro. Perhaps the producers could not resist the spectacle of the Von Trapp father (Plummer had made The Sound of Music seven years before) playing the conquistadore.

The play's success guaranteed Shaffer's future as a playwright - and started a fruitful relationship with Dexter, who also directed his equally tricksy plays Black Comedy (where dark and light are reversed) and Equus (with a cast formed of horses). But the production of The Royal Hunt of the Sun had a wider significance. As Barker wrote at the time: "What especially causes the heart to sing is that this time our National Theatre is not reviving a classic. It is presenting a new play by a young author, ungrudgingly, on a literally dazzling scale."

Another stage production:

from Next Stage