Carter, played by Emily VanCamp, was last seen in Captain America: Civil War, and VanCamp has often been questioned on if and when she would be seen in another Marvel caper.
In an extensive interview with Collider, writers Christopher Markus and Stephen McFeely admitted that she did feature in an early draft of the script – but was soon cut.
Markus said: “[She was] in previous drafts, a couple of times.
“Sharon Carter was a victim of what we came to realise as we were writing it: if you’re writing scenes that are setting up people before the story has got to them – which is Steve and Sharon trying to make it work in an apartment – they’re going to go.
“Because the movie does not have time to catch up on ‘previously in the life of Steve Rogers’.”
McFeely added: “Once it became a smash-and-grab type movie, anything that wasn’t on the main A-plot, a response to Thanos collecting the Stones, mostly got jettisoned.”
While promoting her new show The Resident, VanCamp had ruled out a return in Infinity War earlier this year.
“It’s hard to fit Sharon into [Infinity War], she’s really in Cap’s world, you know what I mean?,” the star told AOL’s Build series.
“I can’t say anything about anything but I can say that!”
She added: “She sort of fits into the Captain America movies, that’s where her story lies.
“But I can tell you it’s going to be an epic two-parter, from everything I hear and know.
“And the Russo brothers are phenomenal, they can do no wrong. I’m excited to see it.”
VanCamp has been keeping very busy: The Resident has recently been recommissioned for a second season in the US.
Avengers 4 is out in 2019.