Chuck Connors initially turned down the role of Lucas McCain because he thought the salary was too low. The show's producers then considered James Whitmore and John Anderson for the role, but when they saw Connors' chemistry with child actor Tommy Kirk in Old Yeller (1957), they realized he was the best choice for the role and made him another offer, this time at a much higher salary. He accepted.
In the pilot for the series, by Sam Peckinpah (originally written for Gunsmoke (1955) but turned down for that show), Chuck Connors' character was named John McCain, not Lucas McCain, and he didn't have a son. It was producer Arnold Laven's idea to make McCain a widower with a son. Also, McCain was originally supposed to have been a dead shot with a pistol. Laven had the idea to use a customized Winchester rifle as McCain's weapon of choice.
In some of the episodes, note that Lucas McCain (without any connection or relevance to the plot) will handle and fire his rifle with equal precision, sometimes with his right hand, and other times with his left hand. An explanation has never been given for this ambidextrous ability.