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 Quand les aigles attaquent (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.
The film somewhat fictionalizes the character of Trotsky's secretary, who, in reality, was called Sylvia, not Gita. She was several years older than Trotsky's mysterious assassin, and was not regarded as beautiful. It is likely that Romy Schneider was cast because she had had a very public love affair with Alain Delon a decade or so earlier, which had ended very badly. Another detail changed by the film was an omission which may have been occasioned simply by the fact that it would have been very difficult to dramatize believably - the real Trotsky had managed to break three of his assassin's fingers in defending himself.
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).
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.