Sure, the cast of Squid Game are now household names around the globe. But when they were first cast in their roles, the stars were a mix of South Korea’s most well-known and celebrated actors like Lee Jung-jae, who plays Gi-hun, Park Hae-soo (Cho Sang-woo) and Gong Yoo (um, slap guy) and relative unknowns like Oh Young-soo, who plays the game’s oldest competitor, Oh Il-nam (aka No. 001).

It’s Il-nam who inspires much of the marbles challenge, which sees the players team up to play a classic childhood game — only to learn that they’re actually competing against their partners. One of them will die by the end of the episode, which is set in the alleyways of a neighborhood Il-nam remembers from his childhood. 

“It is a very cold and heartbreaking game,” production designer Chae Kyoung-sun told Netflix in a featurette about the making of the challenge.

According to creator Hwang Dong-hyuk, the casting of Il-nam was one of the most important pieces of the Squid Game puzzle. “We wanted Il-nam to be unknown so that he would seem like the actual character in the show,” he explained.

He’s certainly a complex character to nail — sometimes he’s super sharp and at the top of his game, and sometimes he’s deeply affected by dementia and his brain tumor. Thankfully, the actor was up to the challenge. 

“I was reminded of my childhood playing the game," said Oh Young-soo, who used to play marbles as a boy. “And when you watch the marbles scene, you can see the falsehood and truth in people. The game reveals a player’s humanity, and especially to Gi-hun, the main character.”