Michelle is one of five suspects named by Coronation Street bosses for the soap's murder mystery, which began when Frank was found dead on Monday evening (March 5).
Speaking on Loose Women today (March 7), Marsh said that Michelle's desire to protect Frank's ex-fiancée Carla (Alison King) puts her firmly in the frame.
"Sally had blood on her hands, but next week, you'll see that I'm slightly more involved," Marsh explained. "It could be me! She's very, very passionate about her family. There's a little dark side to Michelle, definitely."
Marsh added that she is thrilled with how close Michelle and Carla have become in recent storylines.
"I do love the fact that it's Carla and Michelle in the factory," she commented. "That kind of dynamic really works for us. There was a time when Carla and Michelle fought all the time, and actually it's much more interesting now that we're allies.
"Alison is wonderful and we get on so well. She needs to win 'Best Actress' at the Soap Awards."
Michelle, Carla, Kevin, Peter and Sally are the five suspects named by Coronation Street to promote the storyline.
Show chiefs have promised that the killer will be unmasked later this month.
> Coronation Street poll: Who killed Frank Foster?
> Coronation Street spoiler pictures: Carla and Michelle conspire together
> Coronation Street Andrew Lancel teases Frank Foster killer reveal
> Read more Coronation Street spoilers and news
Watch a preview of Friday night's (March 9) Coronation Street double bill in the video below: