The 35-year-old played band member Paul's "first proper girlfriend" Zoe, in a part that saw her frolic around in her underwear.
In unearthed clips from the film, the hitmaker is seen boasting about Holly's character before she flies out to visit him in Los Angeles.
After spotting Paul writing to Zoe, Hannah Spearritt says: "She taught him how to kiss," to which Rachel Stevens replies: "Taught him? Was there an exam or something?"
Holly Willoughby stripped off for S Club 7's 2000 TV movie
Holly Willoughby played a model in the movie about S Club 7
Holly Willoughby in pictures
Thu, January 26, 2017
This Morning host Holly Willoughby in pictures
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Holly Willoughby wows in plunging pink gown at the National Television Awards
She was great, I really thought she was the one!
Hannah then quips: "Yeah, an oral…" John continues: "She was great, I really thought she was the one. It just didn't work out… we split up just after S Club happened.
"She wished me luck, but she didn't think I could ever be as serious about her as I would about music."
As the story progresses, Holly's sought-after alter ego lands an agent and a cosmetics modelling job – after just two days.
Holly Willoughby played Paul Cattermole's girlfriend
Holly Willoughby's role in the S Club 7 film was her first acting job
As her career soars, the mother-of-three is seen in a lacy black bra posing up a storm, but her success comes between them.
The pair break-up as she has no time for Paul, however not before Holly is seen in an array of flesh-baring ensembles.
As well as Artistic Differences, the TV regular appeared in the likes of Agatha Christie's Marple and Lemon La Vida Loca, as herself.