Viewers have seen Kat meeting with a mystery man behind her husband Alfie's back and Wallace had been filming the storyline for three months without knowing who he was.
"The time felt right to finally tell Jessie," a show insider told the paper, after admitting that Wallace finally learnt her on-screen lover's identity last week.
"But she is still one of only a handful of people who knows the answer to the question on everyone's lips.
"[Show producer] Lorraine [Newman] was adamant this story was going to stay quiet for as long as possible. She has done everything in her power to let it play out on screen with total secrecy behind the scenes.
"It's created a real buzz as everyone's been trying to figure out who Kat's been bedding."
The insider added: "Up until now, none of the cast were in the know, not even her on-screen lover. It really has been that cloak and dagger."
Friday's episode saw the range of suspects reduced to just the three Branning brothers – Derek, Max and Jack.
Jake Wood, who plays Max, also revealed that viewers have been asking him whether he is Kat's mystery man at least 20 times a day.
He said: "It would definitely be in keeping with Max for him to be having an affair. He's got a chequered history and women are his Achilles heel. So it would make a lot of sense for it to be him.
"It would be bad timing, though, because he's planning to get remarried!
"I get asked about 20 times a day if it's me. It's a good indicator people are interested in the storyline.
"It's building to a head and there is going to be a reveal. But that really is all I know as I'm still waiting to be told who it is."
EastEnders continues on Monday (September 17) at 8pm on BBC One.