As announced last year, Billy will carry the Olympic torch through Albert Square this summer as part of the torch's 2012 relay route.
EastEnders fans will see Billy take on the task in some brief live footage which will be slotted into July 23's episode of the BBC soap.
Fenwick told This Morning today (April 13): "That's the bit that's really worrying me! When we did the live episode of EastEnders, it put a few million on the viewing figures obviously, because they're going to watch it and they're going to see if you're going to mess up your lines.
"But the few million who'll be joining this time will be seeing if he falls over - what a laugh that'd be! I've been running round the Square training and stuff like that. I'm looking for every pothole down the market that could be filled in! I'm doing my preparation!"
Fenwick joked: "Funnily enough, I've just worked out that the day that Billy's in prison is Friday the 13th. You could put money on that, couldn't you?"
Discussing Ben's guilt, he added: "I think that's what's good about this storyline, because usually in soaps it's more like a whodunit. But this is a bit like the old Columbo - only the audience know who's done it. Will he get away with it? It was really good fun to film, a real proper Mitchell story."
EastEnders continues tonight at 8pm on BBC One.
> EastEnders' Billy Mitchell to carry Olympic torch through Square
> EastEnders' Billy quizzed over Heather's murder - unseen pictures
> Read more EastEnders news and spoilers
Watch Billy's EastEnders character trail in the video below: