Joseph Losey originally offered the part of Leon Trotsky to Dirk Bogarde, with whom he had made five other films. Losey admitted that the script was terrible, but told Bogarde that it would be revised. Bogarde turned the role down, embittering Losey, who felt that Bogarde didn't trust him. Richard Burton, who had worked with Losey on Boom! (1968) did trust Losey enough to take the part, even though he was shown the same script.
Judging by his private diaries, Richard Burton seems to have genuinely thought at first that the film had the potential to be a "blockbuster" on the scale of his recent action hit Where Eagles Dare (1968). He appears only to have read the script after accepting the part "and discovered that almost every scene I do takes place on 'the Patio of Trotsky's house'". His journal records his gradual realization that the static, dialogue-heavy script, combined with the limited English of other actors, would materially harm the film; the set representing Lev Trotskiy's house was stocked with genuine period books and magazines, and Burton's greatest pleasure during filming seems to have been reading these in breaks from shooting.
Charlton Heston had turned down this in 1960, when he was looking for his first lead role after winning the Oscar for Ben-Hur (1959).
In the long sequence of the corrida de toros, surprisingly showing a long part of it, except the moment of the truth, the bullfighter seems to be Mexican matador Rafael Gil Rafaelillo, from Tijuana. A possible reason to focus more on the picador and its violence on the bull, rather than the final killing by the matador, could be that this young torero seems to fail with the sword. Still, even if excessive and long, it seems to work as a sort of metaphor for the assassination which is about to happen.
One of the films included in "The Fifty Worst Films of All Time (and how they got that way)" by Harry Medved and Randy Lowell.