I don't know if Collins was a stunt-cast - was she as well known in the 1960s as she later was?
Pretty well-known, according to Wikipedia
. She was a popular pinup model in the UK. 20th Century Fox signed her in 1955 with an eye to making her a rival to Elizabeth Taylor, and she had the starring role that year in Howard Hawks's cast-of-thousands historical epic Land of the Pharaohs
. She won several "most promising new star" awards in the late '50s, and was the first or second female lead in nearly a dozen films between 1955 and the time she did "City on the Edge." So yeah, I'd say she was a fairly major name at the time.
Robert Walker, Jr. (Charlie Evans) was also a pretty big name in the day, though largely due to his famous actor parents and his studio-mogul stepfather. He'd made a strong impression in his first two feature roles in 1963, but his film career slowed down after his lead role in Ensign Pulver
, which is probably why he was doing TV.
It's worth keeping in mind that TV was a less respected medium than it is today, so there was less crossover of movie actors to TV roles. So if you're talking "big name" guest stars in '60s TV, you're probably talking about either successful TV stars (e.g. Jane Wyatt or Elinor Donahue) or film actors who were past the peak of their big-screen careers. Or people like Ricardo Montalban and John Fiedler who were well-known stalwarts but not (yet) big stars.