The handsomely immobile face, commanding height and stiff manner of William Franklyn, who has died aged 81 of prostate cancer, was ideally suited to playing enigmatic intelligence officers on the large or small screen. One of his most enduring achievements was in the 1960s television series Top Secret, in which his spymaster was the epitome of steel-faced purpose. However, he achieved uncomfortable, if lucrative, fame as the suave man in the “Sch… you know who” TV commercials for Schweppes tonic water.His contribution wrapped an atmosphere of elegance and mystery around a product familiar to the point of banality. This was one of the most successful ad campaigns ever: it occupied him for three weeks a year, made him as much money as a star’s salary in the West End, and was dropped by Schweppes only because it was making Franklyn more prominent than the product. Between 1965 and 1973, he appeared on screen in 10 of the commercials and voiced 40.Born in London, he was the son of Leo Franklyn, a stalwart of the Whitehall farces. Franklyn Sr retired at 79 from No Sex Please, We’re British; he looked like a funny man, but William definitely did not. After an early childhood in Australia, he returned to the UK, and, small and sickly, he forced himself first into parachuting, which led to him becoming a paratrooper in the second world war, and then into acting, because, he said, it also made you conquer nervousness.

Unimpressed, Leo decided his son should be a journalist, but agreed to go and see him perform in the comedy Arsenic and Old Lace on the pier at Southsea. “You’re getting your laughs – you’d better stick to it,” was his verdict.

It was the beginning of the career of an intelligent if not intellectual actor, which was to diversify into films, television series and panel games. He was for a time a job-guessing panellist on the BBC’s What’s My Line? quiz programme, made notorious by the irascible Gilbert Harding, in which Franklyn always remained calm. He called it “an acting job until you get relaxed into it”.

At one point, any title with the word “spy” in it obliged producers to telephone his agent. In the late 1970s, after he had survived the Schweppes connection for two years – he was dropped because producers still thought him too close to that role to offer him anything else – he was offered the role of host, quizmaster and interrogator on Master Spy. This was an ITV series in which members of the public were asked to decypher coded messages, disguise themselves, talk their way out of situations, outsmart foreign agents and identify a visiting celebrity wearing disguise. There were also appearances in The Avengers and The Scarlet Pimpernel.

In Pit of Darkness (1962), one of the fairly few cinema films in which he took the starring role, Franklyn was a husband manipulated and compromised in a murder investigation by mysterious conspiracies. Mercilessly exposed as the central player on the big screen, and playing a distracted part for which audience sympathy was vital, he seemed undemonstrative, even wooden.

Later, as a change of gear, he appeared in another television series, Paradise Island (1977), in which he played the entertainments officer of an ocean liner, who shared, after the ship went down, a desert island with Bill Maynard’s puritanical cleric. Neither this nor Master Spy enjoyed the laurels conferred on Top Secret, but they confirmed his reputation as an actor who could get laughs, without being a natural funny man.

Then, after a run of such West End comedies as There’s A Girl In My Soup and Tunnel Of Love, plus many film roles, his acting hit a dead patch. He bought himself a barrow and went from house to house in fashionable areas of London asking for “junk”, which he then sold as antiques from a shop in the Portobello Road. He was proud to say that when he started he had a bank overdraft of £1,700, whereas, when he stopped, as acting work reappeared, he was double that amount in credit.

He was even prouder of his work as a director, beginning with There’s A Girl in My Soup with an Italian cast (who spoke no English) in Italy. Franklyn took a six-week Berlitz crash course in Italian and returned from the production with anecdotes, carefully polished over the years, about how as a director of Italian actors it did not matter whether you spoke Italian or not because they chatted among themselves and took no notice of the director.

Franklyn’s stiff-upper-lip English persona (he loved cricket, squash and tennis and once tried to play for Essex as a fast bowler) was a durable one. In 1991, after appearances in 66 television shows and an eight-year absence from TV, he appeared again, this time as a smooth Tory nastie in Alan Bleasdale’s social drama series GBH. He was a judge in The Courtroom (2004), and in 2004-05, he was the voice of the Book in The Hitchhiker’s Guide to the Galaxy on Radio 4. His image may have been limiting, but it commanded a definite niche.

He was married first in 1952 to Margo Johns, by whom he had a daughter, and then to Susanna Jupp, by whom he had two daughters. She and his daughters survive him.

· William Franklyn, actor, born September 22 1925; died October 31 2006