The A Knight's Tale actor tragically passed away from an accidental prescription drugs overdose in 2008 aged 28, and the star's life is set to be explored in the upcoming project, which left his sister Kate in floods of tears after watching just a short clip.

She told The Project: "I think I screamed and cried through the whole first viewing."

Heath and his sister Kate attend a premiere. Photo / Getty
Heath and his sister Kate attend a premiere. Photo / Getty

But Kate believes the show will prove to be a "cathartic experience" for her, as well as Heath's former partner Michelle Williams and the one-time couple's 11-year-old daughter Matilda.

She said: "It's quite a cathartic experience, I think, especially for our family."

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Kate has revealed "none of the footage" included in the feature-length biography, which features rare videos of the golden-haired hunk at home and on set at work, as well as interviews with his friends and colleagues, has originated from Heath's family.

Speaking about the footage that features, Kate said: "None of the footage that's shown in the film actually came from the family."

Michelle Williams and Heath Ledger. Photo / Getty
Michelle Williams and Heath Ledger. Photo / Getty

And she has claimed she "didn't know" she would be included in the production, which has been directed by Derik Murray and Adrian Buienhuis.

She added: "In fact, I didn't know it was in there until I saw it myself."

Although Matilda was only two when her father died, Kate regularly shares fond memories of her brother with her niece.

Kate previously said: "When [Matilda] picks up her pencil, it reminds me of Heath; when she walks, it reminds me of Heath; when she gets on her skateboard, it reminds me of Heath.

Family of late actor Heath Ledger, father Kim Ledger (C) and mother Sally (L) and sister Kate (R). Photo / Getty
Family of late actor Heath Ledger, father Kim Ledger (C) and mother Sally (L) and sister Kate (R). Photo / Getty

"I tell her about her daddy every time we see each other. I tell her little stories of him growing up and how he used to chase me with the cricket bat."

I Am: Heath Ledger was initially set to air earlier this month, but has since been pushed back to hit the screens on the American broadcasting network Spike on May 17.