Indigo 'Indi' Walker (Weaving) will shortly be offered a new job, which prompts boyfriend Romeo Smith (Mitchell) to pop the question after fearing her departure.
Mitchell told TV Week: "The fact that Indi's been offered a job brings up the prospect of being separated from her again.
"And Romeo doesn't deal with that well. He's finally got the girl of his dreams, and she might be going away."
Weaving added that Indi's acceptance to the proposal is "beautiful", but that filming the scenes themselves were strange for the actors.
"They're having a bite to eat and there aren't many people there. Indi is shocked, but also just overwhelmed with happiness," she said.
"Luke and I were just freaking out because it was so weird. We used our nerves to our advantage, though, because obviously neither of us has been proposed to before."
Romeo has been involved in an infidelity storyline in the soap, after having an affair with Ruby Buckton, played by Rebecca Breeds.
Mitchell previously described on-screen kissing as "weird".