The actress, who will marry Hollyoaks star Jamie Lomas in September, said that Michelle will take a "no-nonsense" approach to the drug use.
Marsh told Inside Soap magazine: "She wants to kill him! She's so angry and ashamed, especially as he seems more concerned about the drugs than the fact he's been caught taking them.
"He drops them on the floor and scurries to pick them up. It all gets too much for Michelle. She feels like an idiot and begins to see Ryan as a stranger."
Marsh confirmed that Michelle goes as far as to report his drug use to police, with the 36-year-old arguing that "she's in a state of panic and doesn't know what else to do".
She added: "Michelle takes no nonsense and wants to show people she isn't shying away from her problems. She's tackling them head on.
"Her biggest fear is that Ryan's putting himself in danger, risking everything - including his life."