Author and YouTuber John Green thought his breakout bestseller wouldn’t be a commercial success, wrote 40,000 words for one sentence, and brought Steve to tears.

Suleika Jaouad was diagnosed with cancer at 22. She made her illness the subject of a New York Times column and a memoir, "Between Two Kingdoms". She and Steve talk about what it means to live with a potentially fatal illness, how to talk to people who've gone through a tragedy, and ways to...