Early episodes included the gimmick of having Sir Roger Moore speak directly to the audience. Later, this was replaced by narration. In one early episode, an old woman guesses Templar's name ... she says his name must be James Bond. Sir Roger Moore, of course, later went on to play James Bond 007.
Among the actors offered the role of Simon Templar was Patrick McGoohan. McGoohan turned down the role owing to his disapproval of Simon Templar's womanizing (he also turned down James Bond in Dr. No (1962) for much of the same reason).
A version of the car story seen elsewhere says that Jaguar were indeed requested to supply the (then new) E type as an ideal "typically British" steed for Simon Templar, also typically British in the early sixties, Jaguar were bedeviled with strikes and parts supply, and could not deliver on time. Commencement date was looming, and finally Sir Roger Moore volunteered his personal car, the now famed Volvo P1800. Although stylish, it was hardly the racy image needed (post-production gave it the exciting exhaust note). For Volvo, it was a godsend. The P1800 had been selling sluggishly in the UK, suddenly it was "cool", and sales skyrocketed, and as a result, production was extended past the formerly planned finish date. On the rare occasions the vehicle was actually on-location on real streets, it was technically illegal, as the "ST 1" licence plate was registered to another vehicle (the cops turned a blind eye).
During the writing of his manuscripts, Leslie Charteris constantly designated Simon Templar by his initials (S.T.) in order to save time. That's how the idea came up to give him the nickname "the Saint".
Sir Roger Moore was originally told that the series was going to be a half hour. It wasn't until they held a press conference to announce the series, that he found out it was going to be a one hour series.
The producers of the show went to Jaguar to ask for a free car, in return for the publicity that would follow a successful television series. Jaguar refused, so the producers went to Volvo which was more than happy to risk one P1800 coupe, and the publicity, as it turned out, far outweighed the value of the car.
According to Sir Roger Moore in his autobiography, Producer Robert S. Baker never gave Leslie Charteris script approval. He only had the right to comment on them. The short stories, on which many of the episodes were based, had a first and third act, but the show's writers had to go back and add a second act, in order to fill the entire hour.
The series was one of two similar hopefuls the production company had lined up for the 1962-63 season. The other was Man of the World (1962), which had cost more money, but had a different style that did not catch on, and was cancelled.