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Fame's Kay Panabaker has been cast as a young Kitty, newcomer Anna Wood has landed the role of Sarah, and Kasey Campbell is reportedly in talks to play Kevin.
The episodes will also feature a younger William and Nora Walker, plus William's former mistress Holly Harper, but these roles are yet to be cast.
Executive producer David Marshall Grant told TV Guide "[The flashbacks] are all in service of understating secrets of the past that are slowly revealing themselves now."
Last week, it was announced that Cody Longo will portray a young Tommy Walker, a role which Joe Jonas was previously rumoured to be up for.
