The actress's character Mercedes Fisher has recently been seen giving into temptation with Calvin Valentine (Ricky Whittle), the husband of her sister Carmel (Gemma Merna). Their feelings will grow stronger as the plot continues over the coming weeks.
Discussing the twist, Metcalfe is quoted by Bang Showbiz as saying: "It seemed a bit far-fetched at first. We had lots of meetings about how we were going to do it and Ricky felt like Calvin wouldn't do that to his wife Carmel because he's in love with her.
"And Mercedes thinks Calvin is a bit of an idiot. So we all had reservations, but now that we've started filming it and we're getting into it, it's brilliant."
Meanwhile, on her steamy scenes with Whittle, she commented: "It's just like kissing any of my other friends on the show! It's weird. It's like being 13 when you used to kiss your mates for the sake of it, to see what it's like.
"It doesn't feel like a snog. You're doing it because you have to, not because you fancy them."
Metcalfe added that she expects viewers to think Mercedes is a "bitch" throughout the affair storyline.