Jack P. Shepherd says David Platt and Nick Tildsley wouldn't "think twice" about betraying each other.
The 'Coronation Street' siblings are currently held together by a secret after Nick (Ben Price) scammed their grandmother Audrey Roberts (Sue Nicholls) out of £80,000 and used the money to set up a barber's shop.
Speaking to Digital Spy, Jack said: "At the minute they're the best of friends and they're in it together, but if push comes to shove, I don't think either of them would think twice about throwing the other one under the bus."
The soap star opened up about the brother's history, and hinted at both of them having a "darker side", even if they are both in the situation together.
He added: "David was horrified by what Nick had done at first. I think Nick also regrets doing it, actually. But here they are and they're trying to get on with it. I think they saw an opportunity and they both just went for it and now they have to live with that.
"David's always had that dark streak in him. Nick, you could argue, now has a darker side due to the brain injury David gave him.
"The writers can delve into that when they want to give Nick a reason for doing something bad, as he was always the golden child before. I think both of them do regret it, but ultimately if they can get away with it, they will."
His comments come after co-star Ben compared the brothers to biblical siblings Cain and Abel, and admitted while they "fight and hate each other" but will always be bonded by "blood".
He recently said: "It's like a Cain and Abel situation - it's blood. If you have a strong family, you can do whatever the hell you like to each other."
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