When Isabel Wilkerson was deciding how to shape her book The Warmth of Other Suns, she held casting calls for the roles of protagonists.

Interviewing 1,200 people, about 30 fit the bill from which she selected three: Ida Mae Gladney, who left rural Mississippi for Chicago in 1937 with her husband and two small children; George Starling, who departed Florida’s orange groves on a train in 1945 for New York City, and Robert Foster, a young surgeon who drove his Buick out of Louisiana to California in 1953.

Through their life stories, she explains the Great Migration of 6 million black Americans between 1915 and 1970 and how it shaped our nation.

Leaving Virginia tobacco farms, rice plantations in South Carolina, and Mississippi cotton fields, they fled north and west as if under a spell or a curse, she writes in her National Book Award-winning work, subtitled The Epic Story of America’s Great Migration.

“From the early years of the twentieth century to well past its middle age, nearly every black family in America, had a decision to make. … In this, they were not unlike anyone who ever longed to cross the Atlantic or the Rio Grande.”

Wilkerson, 54, will speak at 7 p.m. Wednesday in the Main Library’s McMaster Center for the Authors! Authors! series presented by The Blade and the Toledo-Lucas County Public Library. It will be her first visit to Toledo, and she’s excited.

“Ohio was one of the great receiving stations of the Great Migration, one of the states that people dreamt of,” she said. “Whenever I’ve been to Ohio, conversations go to the state of Alabama, which sent so many people to Ohio.”

Her 622-page book won a slew of literary honors, was a book club favorite, and was President Obama’s choice to read on his 2011 Martha’s Vineyard vacation, after which he penned her a letter.

Warmth has been her life for 20 years — 15 researching and writing, and another five speaking about it since its 2010 publication.

“This book has had me on the road from Amsterdam to Alaska, Singapore to Italy, and almost every state because the Great Migration resulted in people going to every state from the South,” she said.

Last year the U.S. embassy in Singapore invited her to speak to various groups. Singapore, she noted, is a land of immigrants.

“To be human is to migrate: that’s how the United States largely became populated, whether by voluntary or forced migration.”

The impact of such migration can be hard to imagine because it is so much a part of our lives. But consider how it affected voting, particularly in Ohio, Michigan, Wisconsin, New York, Illinois, Pennsylvania, and California.

“Even blue and red states tend to reflect the arrival to these northern states of millions of people, who were new voters. Before the Great Migration, the vast majority of African Americans were not permitted to vote.”

And consider the North’s cultural life without Motown, rhythm and blues, numerous singers, actors, authors, athletes, and the president’s wife (but not the president).

Ms. Wilkerson lives in Atlanta. She loves jazz, digging her hands into soil, and being on the road, which feeds an insatiable curiosity: “Seeing new places, new ways of being, new ways of seeing how human beings live out their lives and work through their challenges.”

She grew up in Washington, the only child of Alexander, a civil engineer, and Ruby, a teacher, both graduates of Howard University, as is she.

As a reporter for the New York Times, she won the 1994 Pulitzer Prize for feature writing for a trio of articles, including two on flooding in the Midwest. Shortly thereafter, she began working on Warmth, supporting herself by writing occasionally for the Times, teaching, receiving a book advance, and in 1998, a Guggenheim Fellowship.

The story she was compelled to tell is both warp and woof of her internal fabric. “It’s my family’s story, the story of everyone growing up around me, the story of the majority of African Americans in the North and the West. It had been working its way through my subconscious most of my life. It grew out of so many different places it’s hard to imagine where it came from.”

To find her three main characters, one for each migration route, she went to several cities and visited senior centers, churches, and club meetings. Her threesome were in their 70s and 80s, and time, she realized, was not on her side, particularly when they became ill.

In Janet Maslin’s New York Times review she said the book’s greatest strength was its myriad examples of segregation: “It’s one thing to know that southern blacks faced bias in all aspects of their lives. It’s another to know that when an esteemed black doctor from Louisiana needed to perform surgery on a black patient, he couldn’t do it in a white hospital. Driving around with his own portable operating table was easier.”

Ms. Wilkerson expects the book will become a screen presentation at some point. Students of history may enjoy her Facebook page, which she updates several times a month with essays about people and events.

Her poignant Dec. 14 post begins this way: “Seventy years ago, in 1944, George Stinney became, at age 14, the youngest person executed in the United States in the past century. He was so slight of build [90 pounds] that props were required to fit him in the electric chair. He had been convicted of first degree murder after a 2½-hour trial and 10 minutes of jury deliberation in a textbook case of Jim Crow justice.”

Tickets for Isabel Wilkerson’s 7 p.m. Wednesday talk are $10, $8 for students, at library branches and at the door. Seating is first-come, first-served and is limited. Her book will be available for purchase and she will answer questions and autograph books after the talk. The Main Library is at 325 N. Michigan St. Information: 419-259-5266.

Contact Tahree Lane at: tlane@theblade.com or 419-724-6075.

First Published March 15, 2015, 4:00 a.m.