Hulu’s “We Were the Lucky Ones,” based on the book by Georgia Hunter, tells the true story of the Kurcs, a Jewish family who lived in Poland at the onset of World War II. The series follows the Kurcs as the events of the war and the Holocaust separate parents and children, tearing families apart.

As casting director Fiona Weir read the script for the first episode, showrunner Erica Lipez stressed that despite the grim setting, it ultimately was a celebratory story — and it was important she get the family dynamics right.

Speaking with Variety, Weir says Hunter, who based the book on her family experiences, turned out to be her most valuable source. “These were real people that she either knew or discovered, so there was lots to talk about in who these people were. And then, we had to create a group that organically felt like a family,...