According to the reliable "David Janssen Archives", Janssen was offered the lead in three series in 1957: "The Californians", "Pony Express", and "Richard Diamond".
Janssen would have apparently played the Adam Kennedy role of crusading Irish newspaper editor Dion Patrick on "The Californians". When the format of the show was revised, Kennedy was replaced by Richard Coogan as San Francisco's top cop. Art Fleming (the original "Jeopardy") co-starred in the Coogan episodes.
Adam Kennedy later got one of the starring roles on the entertaining daytime soap opera "The Doctors" in 1965. Kennedy played dynamic inventor-entrepreneur Brock Hayden. Hayden was eventually killed off on screen, perhaps because he wasn't a doctor and was difficult to fit into the stories. I remember I was shocked when Brock died on the operating table after being shot. Kennedy played opposite Ellen Burstyn on "The Doctors".
Kennedy had studied acting at the Neighborhood Playhouse in New York with Sanford Meisner.
Adam Kennedy later wrote 20 novels. One of the novels was made into a move by Stanley Kramer: "The Domino Principle" starring Gene Hackman, Candice Bergen, and Richard Widmark. Kennedy wrote the screenplay for that film as well as a couple of other films.
Kennedy was also an oil painter. Quite a guy.
James Best got the lead in "Pony Express" after Janssen turned that down, but it didn't sell in 1957. In 1959 it sold in syndication with Grant Sullivan in the lead.
Janssen wisely chose to become Richard Diamond. It was his first starring role and it made him a star. Janssen was recommended for Richard Diamond by the great film director William "Wild Bill" Wellman. Wellman had just finished directing Janssen (and Clint Eastwood) a few months before in "Lafayette Escadrille").