According to the DVD commentary, neither Sir Roger Moore, an uncredited co-Producer, nor Robert S. Baker, the credited Producer, ever had a contract, other than a handshake with Lew Grade. They produced the entire twenty-four episodes without a single written word guaranteeing that they would ever be paid.
Throughout the series, Danny Wilde (Tony Curtis) is almost always seen wearing gloves. According to DVD commentary, this was a gimmick developed by Curtis, to make his character unusual, and therefore create some buzz for the series.
In West Germany, Switzerland, and Austria, where this show was wildly popular, this series was dubbed in an irreverent way, ad-libbing a lot, thus departing from the original scripts, and using very funny, often absurd colloquialisms and phrases. (For example, the speakers even improvised jokes about their own dubbing work in the process). Because of that, in West Germany, it had more of a comedy element to it, more so than its original version, making the characters even more quirky and lovable. The West German dubbing owes a big part of this show's success and popularity in West Germany, Austria, and Switzerland. (The same thing happened there with the dubbing for Starsky and Hutch (1975)). Other countries, such as France, followed the West German model. Tony Curtis even asked Rainer Brandt, the West German speaker, to write the dialogue for the next season, but the series got cancelled before this could happen.
The most ubiquitous firearm used in the series is a Model 1934 Beretta in 7.65 Browning, a.k.a. .32 ACP. It appears in almost every episode involving a handgun. When a revolver shows up, it is almost always a Colt Detective Special.
Popular in Denmark, and still repeated on television forty years after. The Danish title "De uheldige helte" (The Unlucky Heroes) plays with the words held=luck and helt=hero. The title tune was also a bit of a hit.
In West Germany, and some other European countries, this series was dubbed in an unconventional way, using very funny, often absurd expressions and phrases. Because of that, it became very popular (unlike the English version) and is still regularly repeated on German television today.
The filmed sequences, presenting Danny Wilde and Brett Sinclair, in the opening credits, were not filmed expressly for said credits, but were taken from various early episodes (all of them in southern Europe), mainly The Persuaders!: Overture (1971) (the two men racing their sports cars, Tony Curtis and Sir Roger Moore distracted by a blonde in a bikini walking between them, Brett flirting with two beauties) and The Persuaders!: Powerswitch (1971) (the water-skiing shots, and Danny shown as a businessman at work at a desk).
Tony Curtis was born Bernard Schwartz of Jewish-Hungarian immigrants, and if you pay attention, you will find him sprinkling inside jokes to his origins throughout the series. In The Persuaders!: Greensleeves (1971), for instance, his character pretends to be Brett Sinclair's new butler, Grzegorz (Gregor), and explains his (fake) east European accent by his being from a "Hun-GAA-rian" from "BU-dapesht" (unfortunately for Danny Wilde, the black beauty from an African republic, with whom he shares this, speaks the language perfectly). Torn from his Paris hotel shower in The Persuaders!: The Old, the New, and the Deadly (1971) by the telephone ringing, which turns out to be a wrong phone number, a dripping wet Danny states, "No, this is not Mr. Schwartz, you got the wrong room!"
In episode fourteen, "The Man in the Middle", Brett Sinclair accidentally dropped his passport during a fight in the first scene. As part of the plot, it is picked up by MI5 Agent Kay (Suzy Kendall), and opened at the photo page in close-up. Lord Sinclair's place of birth is clearly stated as "Scotland".