Marlo Thomas and Phil Donahue wanted to do something special to celebrate their 40th wedding anniversary. They could have planned an exotic trip or a big party, but instead, they wrote a book together. In What Makes a Marriage Last: 40 Celebrated Couples Share With Us the Secrets to a Happy Life, the pair asked other famous, longtime married partners their tips for a successful union. 

“Believe it or not, that’s a subject that doesn’t get a lot of attention,” Phil tells Closer. “All you read about is how many people get divorced!” Long marriages, the pair discovered, are strengthened by trust. “It’s not just trusting that you won’t betray each other, which is very important, but also trust that what matters to you also matters to them,” Marlo says. “You build  trust when you realize that this person always has your back.”

When Marlo, 82, and Phil, 84, wed in 1980, there were growing pains. Marlo was working as an actress in L.A., while Phil raised four sons from his previous marriage and taped The Phil  Donahue Show in Chicago. “I had an aunt tell me, ‘that’s not marriage’  because we didn’t live in the same city all the time,” Marlo says. “But the thing I learned is that you define what your marriage is yourself.”

American Icon Awards Gala, Arrivals, Beverly Wilshire Hotel, Los Angeles, USA - 19 May 2019

It wasn’t easy. In those first years, Phil grappled with feelings of jealousy about his actress wife. “She was doing a movie with Kris Kristofferson,” he recalls. “I got over it, and I really felt so much better for having lost my insecurity.” 

Marlo, who had enjoyed the single life until she wed Phil at age 42, also experienced a period of adjustment. “I married someone who had four sons who lived with him. He belonged to a lot of people,” she says. “I dove into it because I loved Phil and his kids are great, but it was quite a challenge for me.”

While researching their new book, Marlo and Phil were tickled by the different ways other couples have learned to live together in harmony. When Ron Howard and his wife Cheryl disagree, they each tape-record a two-minute argument presenting their side for the other to listen to. Bob Woodward and his wife Elsa say “I love you” every night before turning in, no matter what. Bryan Cranston and his wife Robin have committed to attend couples therapy together whenever one of them feels their marriage needs a tune-up.

The common thread among all the couples they spoke with ⁠— and Phil and Marlo, too ⁠— is a commitment to remaining together. “When the going gets tough, you don’t look to get out. You find a way to stay in,” says Marlo. “I thought Kyra Sedgwick said it so concisely. She said, when you get married you can’t go in with a plan B. There’s no plan B!”

For this story and more, pick up the latest issue of Closer Weekly. on newsstands now!