John has returned to the BBC One soap as Christian Clarke for this week's 30th Anniversary celebrations and has revealed that the cast are finding out the unfolding plot in real time.
Last night, viewers got their first glimpse of the stars working live in front of the cameras and John has admitted that nerves were at an all time high backstage.
Appearing on Good Morning Britain, he said: "Nobody knows who killed Lucy - we are finding out in real time.
"So last night there were live inserts going in and, at the end of the episode when Max dropped that cliffhanger, we were like, 'I can't believe it! No, no!'
"Absolutely there are nerves backstage and it is so tense, but this is also because we're finding out the stories there and then. You've got double tension!"
John appeared in the 25th Anniversary live show, which saw Stacey Branning confess to the murder of Archie. And at the time, John guessed she was the killer!
When asked about this week's mega mystery, he replied: "Until last night, I did have somebody in mind but now it's changing. First I thought I knew, but I've honestly got no idea. If I had to choose one, I would say Ian."
He added: "If we knew who had done it, it would get out there somehow. The fact that they have kept this secret is fantastic. It's going to twist and turn as the week goes on!"
Viewers last night saw Max Branning tell his daughter Abi: "[Lauren] knows... that you killed Lucy."
However, as the scene was filmed live, actor Jake Wood has been forced to clarify his lines, pointing out he said YOU not WHO to his co-star Lorna Fitzgerald.
Who do you think killed Lucy? Tell us below, as the best comment from across the website each week wins £50!