Regis Toomey, Zasu Pitts, Jay C. Flippen, and Sir Cedric Hardwicke all started out in silent films. This is an extremely high percentage of actors on an episode of a television show in the mid-1960s.
The following all received 'Special Guest Stars In Alphabetical Order' credits Elizabeth Allen, William Bendix, Bruce Cabot, Rod Cameron, Fred Clark, Jay C. Flippen, Cedric Hardwicke (as Sir Cedric Hardwicke), Stephen McNally, Suzy Parker, Zasu Pitts and Will Rogers Jr..
Barry Kelley's prominent role as Lt. Nolan suggests he was intended to be a regular, but he appeared in no further episodes.
In this pilot episode, the "special guest stars" include actors who did not play suspects in the murder case. This would soon change.