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 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 rocketed 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).
Early episodes included the gimmick of having 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. Roger Moore of course later went on to play 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).
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".
According to 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 that many of the episodes were based on 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 producers of the movie went to the Jaguar Company 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.
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 canceled.