I'm not knowledgeable about Hollywood use, and I find no other instance in a quick Google search.
But in the 19th century, when theatre was the principal dramatic medium, there was a distinction in the UK and US between actors who worked in the theatrical centers, London and New York, and actors who spent their careers in local or touring stock companies without ever being seen by metropolitan audiences. A 'country' actor was one of the latter; and you'll find occasional references in old theatrical memoirs to so-and-so being a 'good country actor'—that is, an actor who was respected in The Profession despite never having made a mark on the West End or Broadway.
This isn't directly applicable to the use here; Widmark had paid his dues in radio in the 40s, but by the time Two Rode Together was shot he was, if not quite in Jimmy Stewart's league, an established Hollywood star. I suspect Ford was playing director games with Stewart, warning him that Widmark was a master of his craft and Stewart had better be on his toes or Widmark would steal the scene.