It was confirmed earlier this month that Smart will play Jasmine, a "pushy and sexy" friend of Rossum's character Fiona in two future episodes.
"She will be coming in towards the end of the season," Rossum told the Los Angeles Times. "She will be a new character; her character was not in the British series. She'll play the mother of a child in Debbie's class."
She added that Jasmine will be "very perky and everything Fiona is not".
"She meets Fiona and befriends her in a very kind of overly friendly way," she explained. "She's just very interested in being friends with Fiona, and [Fiona's friend] Veronica is not happy with that."
Shameless continues on Sundays at 10pm on Showtime.
> Amy Smart wins Shameless role