Heidi Klum promises 'good surprise' for this year's Halloween costume

Heidi Klum has promised a "good surprise" for this year's Halloween costume.

The 51-year-old actress loves the spooky season and goes all out with her outfits for her annual party — with recent efforts including a worm and a stunning peacock — which has been referenced in her new campaign relaunching Unicef USA's Little Orange Box to help Trick-or-Treaters collect donations as they go door to door.

Asked if she could dress up as the box itself this year, she told People magazine: "Hmm... let's just say you'll have to wait and see! You know I love a good surprise."

Heidi admitted the "fun" each year is doing her best to "outdo" her previous costumes, which bodes well for this year's offering.

She added: "Every year, I go all out with my costumes, so each one holds its own special place in my heart.

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"The fun is in trying to outdo myself every year, but what really makes it memorable is the excitement and joy of the whole Halloween experience - there's just nothing like it."

In the new The Box Is Back campaign, Heidi appears with Project Runway designers Erin Robertson and Kentaro Kameya, and Making The Cut duo Ciara Chyanne and Jonny Cota.

She tasks the quartet with helping her come up with this year's costume, calling for something "timeless" and "retro", before telling Jonny: "Think small but with a big impact."

Ciara suggests a piece of turquoise fabric in a similar shade to the Unicef logo, but Heidi wants it to be "more orange".

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A young boy chips in and suggests Heidi could dress up as the Little Orange Box.

The model says: "The box is back. Add some meaning to your Halloweening with Unicef."

Heidi had a blast working on the campaign, having "so much fun" for a good cause.

She added to People: "We laughed, reminisced and had so much fun bringing this PSA to life. It's always amazing when people you love working with come together for a cause that means so much."

ALSO READ: Heidi Klum takes 6 hours and 10 Cirque du Soleil artists to create Halloween costume