(A/N: The first line of this is actually in the show [but it's not in the movie], so I just thought I'd mention that so I'm not taking credit for something I didn't write. I hope you enjoy the story!)
Mistake
"Now, Mr. T, you do love me just a little bit, don't you?"
Sweeney's way of dealing with Mrs. Lovett talking like this is to pretend she's Lucy. The two women are nothing alike—Mrs. Lovett is brunette, outgoing, loud, while Lucy was blonde, introverted, and quiet—but Sweeney's good at pretending.
"Of course," he replies. Sometimes he adds in an "Anything you say" for some variety.
Once, he almost calls Mrs. Lovett 'Lucy.' She's jabbering away about her dreams of the sea, and he mutters an "Of course, Luc—" He stops abruptly, realizing what he has done.
Mrs. Lovett's eyes narrow. "What did you just call me?"
"I called you Mrs. Lovett," Sweeney lies quickly. He's not sure why he doesn't just tell Mrs. Lovett the truth, or why her feelings matter so much to him, but he just can't bring himself to admit his imagining to her.
"You were going to call me Lucy, weren't you, Mr. T?"
"What if I was? You think just because Lucy's dead now, I can start loving you? Well, you're wrong!"
"I'm just saying, I think it's time you moved on. It's been fifteen years. Lucy would want you to be happy…with me."
This makes Sweeney snap. He pins Mrs. Lovett against the wall, glaring at her. "You listen to me, Nellie Lovett. Lucy would never want that. I will never move on. So just give up your dreams, okay?"
Mrs. Lovett doesn't wriggle or try to free herself, just glares back at him, repeating one of her favorite questions. "You do love me just a little bit, don't you?"
Sweeney doesn't pretend this time. Mrs. Lovett's feelings don't matter anymore. "No."