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The 41-year-old actress beat Paula Abdul for the guest starring role as a Mode magazine office temp on the ABC series, Entertainment Weekly reports.
She said: "I've always considered myself a poor man's Paula Abdul. [My character Helen is] Amanda (Becki Newton) in ten years. She's still trying to get into the right clubs, wearing tight dresses… tragic."
She added: "I think [she] sees herself as Samantha from Sex And The City. Except, of course, she's a temp."
It was reported earlier that Betty star America Ferrera was spotted shooting a scene for the forthcoming season dressed in a hot dog costume.
Johnston will appear in the second half of the two-hour Ugly Betty fourth season premiere on Friday, October 9 on ABC.
