Gail came into the show as a minor character- she was a friend to Tricia Hopkins who was meant to become a major character but only lasted a couple of years. The irony is that Gail has lasted a lot longer than she was probably intended to.
Betty Hotpot- arrived as the sister of shopkeeper Maggie and took bar work at the Rovers where she has been for 35 years.
Bet Lynch- had a short stint as a factory worker in 1966 but didn’t last long. A new producer took over in 1970 and wanted Julie Goodyear on board as she was impressed with her other work. When she realised that she had played Bet Lynch that character was brought back and spent 25 years in the Rovers.
Emily- was an assistant at the Glad Tidings mission but has remained on the street some 35 years after that establishment was demolished.
Jack and Vera- Vera first arrived as a factory worker in 1974 but Jack did not appear until 5 years later at Gails wedding although Vera did mention him. He became a regular in 1981 and they moved onto the street in 1983 and have been there since- apart from a stint at the b+b.
Ivy- first appeared at the mail order warehouse which was destroyed in 1975 and killed her colleague Edna Gee. She became a permanent character in 1978 when her family were introduced- although this conflicted with her earlier family background.
Rita- made an appearance in 1964 and came back in 1972 and has been there ever since.
Mavis- appeared briefly in 71 and came back the following year and worked in the Kabin for the next 25 years.
Deirdre- made a brief appearance in 72- as a pretty teenager (without glasses) and came back the next year and three husbands and four Traceys later is still with us.
Blanch- arrived for Deirdre’s non wedding to Billy Walker and has been around on and off ever since.
Audrey- came to the street for Gails wedding and stayed a while. Then came back and married Alf nad has been there ever since.
Martin- started in the café and now approaching his 20th year on the street.
Sally- one of kevins many girlfriends at the time but she has lasted the long haul despite both of their by now annual affairs.
Alma- made a brief visit to her café in 81 and came back a few years later and stayed quite a while.

Any more you can think of?

Ivy first appeared in 1971

yes she got the role based on her performance on the Ken Loachs movie 'Kes'.

"Hand off c*cks and on socks". The opening line to Kes.

Audrey started off by appearing in only a few episodes back in April 1979, was Gail's mum, but became a regular in 1985.
Im sure that Betty and Sally were both meant as regular cast from their arrival, Sally certainly was in it a lot when she first arrived.

Deirdre, Gail, Mave, The duckworths, Ivy, Alma as you said were all short term cast at first.

In those days, they was no big fuss when a new character joined or even when they left. Their were no major spoilers as such and when someone joined you had no idea how long they would stay for. perhaps it was better that way.

The same can be said for the present days Sonia Marshall. She's been in it ages for a minor character.

Sally Whittaker originally auditioned for the part of Sue of the ill-fated Clayton family, who only lasted less than a year in the show.

The Street's new producer John Temple was impressed by Sally Whittaker and told the writers to create a character who could "steal Kevin Webster's heart"...and we got Sally.

Sean!!!!

He's only recently come in as a minor character - but i hope he stays around for ages

Same with Violet. She has come in as a minor character but it seems like she has been it it ages!

Yeah, I'm not sure what it is about her. She has a face that seemed to fit in straight away

I don't think Violet is a minor character. Or at least I hope not!

It's the history that's made Violet fit, but I also agree about her face fitting.

Yeah, but she has been brought in as kind of a minor character, but by the sounds of things, this is going to change.