So, before going to Cho Seungwoo, the script has gone to Bae Doona first. When Bae Doona reject it the first time, it was not the same Han Yeojin that we know now. Like she said, she couldn't feel the charm of her character. https://twitter.com/Withyakyu/status/1320265875807588352?s=20

Lee Sooyeon was willing to make adjustments, so after some changes they sent the script back to her. Bae Doona gave an input that it would be better if the character has a special hobby or habit. That was how we got Han Yeojin who likes comic books- https://twitter.com/Withyakyu/status/1320265890340896770?s=20

with her cute drawings and presents. I also heard on a podcast that there was supposed to be a set up about Han Yeojin's family, but after some discussions with Bae Doona, they took it out. (The podcaster didn't mention the source so I couldn't check the detail)

Someone in the production team (it might be Lee Sooyeon herself) was persistent to have BDN. BDN saw the potential in the scenario and as a veteran actor, somehow involved in building the set up of Han Yeojin. If I dare to say, she joining also gave influence to CSW's decision.