Cast and crew have recently been busy filming the early stages of the emotional storyline, which will see Lucy die in mysterious circumstances.
The plot will see Lucy's father Ian (Adam Woodyatt) take centre stage as he struggles to come to terms with the tragic loss of his daughter, while the grief of Brett's character Jane will also be explored.
Brett told Inside Soap: "All I can say is that there are some really brilliant scripts, and Adam Woodyatt's performance is outstanding. I think the nation should prepare themselves for total devastation.
"There are scenes being filmed that, if they don't make you cry, you must have a heart of stone. People need to order Kleenex in bulk!"
On Jane's reaction, she added: "Don't forget, Jane brought up Lucy as her own daughter. I think she will be there for Ian, but I'm not too sure how much room Jane will have to be that rock. She's going to be grieving as well."
Jane has been back on EastEnders on and off since making a shock return in January. However, Brett is still committed to filming the next series of BBC One's Waterloo Road, on which she plays Christine Mulgrew.
Discussing her commitments and the chances of Jane dating Masood Ahmed, Brett said: "Jane can't stay long, because I have to go back to Waterloo Road! But in terms of Masood, I think he and Jane worked best when they were just friends.
"Their story was originally based on the 1970s sitcom Butterflies, where there were two people who should be together, but weren't. I found that more interesting - I don't think Jane should go backwards."
Digital Spy Soap Scoop video - hit play below for gossip on more Gloved Hand Killer deaths in Hollyoaks, the reveal of Robert Sugden's shooter in Emmerdale, a drastic decision from Kathy Beale in EastEnders and Alya Nazir's guilt spiralling out of control in Coronation Street.