St Clement filmed her final EastEnders scenes last year and fans saw her character Pat Evans make an emotional exit on New Year's Day, when the Albert Square matriarch passed away from cancer.
Coulson, who plays Carol Jackson, today told This Morning that filming Pat's death and funeral episodes had been an emotional experience.
"They were really difficult scenes to play, because we didn't want Pam to leave," Coulson explained. "So apart from not wanting to lose the wonderful character of Pat, we were mourning the loss of Pam.
"You don't see everybody all the time [at EastEnders] at all, and sometimes you can go for months without seeing somebody. And most of the time you're on this big wheel and when people leave, you go, 'Oh, that's really sad', but you get back on. But actually Pam is a real loss up there."
Pat's friends and family will be seen gathering for her funeral in tonight's episode, which runs in an extended 40-minute timeslot.
However, amid all the emotion, Carol has a big decision to make as David Wicks (Michael French) wants her to leave Walford with him.
Coulson explained: "He is the love of her life, and I think she genuinely believes him this time. She's very gullible, isn't she? And he seems very honest and very truthful."
EastEnders airs tonight at 8pm on BBC One.
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