The actor explained that Billy wants to be more responsible now that he has granddaughter Lola and great granddaughter Lexi in his life, and that he no longer has to look up to his Mitchell cousins Grant and Phil.
"For years he wanted to be [like them], but he's always been the black sheep of the family, or the runt of the litter. I don't think he's as bothered about that anymore," he told PA.
"When Billy was with Honey and his kids he'd do anything for [them] and that was why she left him, because he started thieving and lying, but as a means to an end.
He continued: "That's when his back's against the wall - if anyone threatens him or his family, that's when his Mitchell instincts come out. He's just trying to toe the line all the time now but there'll always be temptation because he's always skint!"
Fenwick added that Billy is likely to clash with his fellow Mitchells during the festive period.
"I think what you will see in the coming months is there'll be a few confrontations Mitchell-wise," he said.
"There's a lot to come out with all of that and so it should be interesting around Christmas."
Fenwick helped secure EastEnders high ratings for the soap's special partly live Olympics-themed episode last month.