"So when we started off, Anna and Elsa were not sisters. They weren't even royal," he said.
Instead, Elsa was a full-blown villain because she had been stood up at the altar on her wedding day, and chose to freeze her own heart - leading viewers to believe she's the one in the prophecy destined to ruin Arendelle.
"We started out with an evil female villain and an innocent female heroine and the ending involved a big epic battle with snow monsters that Elsa had created as her army," Del Vecho explained.
He added that in response, Prince Hans would create an avalanche to destroy the city, and Anna convinces Elsa to save everyone. Elsa suddenly realises she's not the subject of the prophecy - Prince Hans is - and her heart unfreezes.
But that storyline just wasn't working.
"The problem was that we felt like we had seen it before," Del Vecho said.
"It wasn't satisfying. We had no emotional connection to Elsa - we didn't care about her because she had spent the whole movie being the villain. We weren't drawn in. The characters weren't relatable."
Eventually, the movie's creators transformed the script into good versus fear, instead of good versus evil - and Del Vecho knew they'd nailed it.
"When the final piece of the creative puzzle fell into place, that moment was downright, well ... heart warming."
