Show bosses recently announced that Kat will be cheating on husband Alfie with a mystery man throughout the summer, and viewers see the plot kick off next week as the secret relationship begins.
The identity of Kat's lover will be kept under wraps for a number of weeks, with even the EastEnders cast and crew currently in the dark over the outcome.
Wallace told Inside Soap of the storyline: "It's really exciting. It's cleverly written so no-one knows who Kat's secret lover is - not even me or the mystery man himself. I love coming in to work every day and discovering even more brilliant twists to the plot."
She continued: "Everyone is going to love the excitement of the storyline. The scriptwriters have been brilliant at putting everyone in the frame. I can't wait to find out who it is."
Next week's EastEnders episodes see Kat appearing close and flirty with the five main suspects in the story - Ray Dixon, Michael Moon and Branning brothers Derek, Jack and Max.
Alfie (Shane Richie) notices his wife's interactions with the men, but has no idea that she has more than harmless fun on her mind.
Wallace added: "You have to remember that Alfie has always loved Kat's sexy side. So he doesn't see any harm in [the flirting]. It's just that Kat doesn't think he has the same interest in her that he used to. Alfie just seems content with domestic life, and the passion has gone."
EastEnders continues on Friday (June 29) at 8pm and then airs five episodes next week on BBC One.