As first revealed last month, Cavanagh's character will discover that she is pregnant in an upcoming storyline. However, her happy news leaves Brian stunned as he had a vasectomy some time ago and hasn't told Julie.
The plot then shows Brian (Peter Gunn) in turmoil as he wonders whether Julie has cheated on him.
However, Cavanagh told Soaplife: "She'd never be unfaithful. And she'd be completely appalled, bemused, betrayed and angry that he could ever think she had been."
Discussing Brian's attitude, she continued: "Julie certainly doesn't get the reaction she expects. She thinks he'll be overwhelmed at the thought of being a dad, but he doesn't even come home that night."
Cavanagh added that she believes Julie and Brian can overcome the misunderstanding eventually.
"She'd forgive him everything, especially now she's carrying his baby," the actress said.
Coronation Street continues tonight at 7.30pm and 8.30pm on ITV1.
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Watch a preview of tonight's Coronation Street double bill in the video below: