Marlo Thomas and Phil Donahue, America’s favorite feminist couple, have embarked on a project together for the first time, a book titled “What Makes a Marriage Last: 40 Celebrated Couples Share With Us the Secrets to a Happy Life.”

The idea came about when actress and activist Thomas, 82, and talk show pioneer Donahue, 84, were mulling over how to celebrate their 40th anniversary. “We said, ‘Do we have a big party?’ ” Thomas says. “Take a cruise? And then we thought there was so much we could learn from long-married couples.”

The two spent nine months traveling to interview each couple in depth, often in their homes. “Everybody put out cheese and crackers and olives and what have you, and it had a very homey, double-date kind of feeling,” Thomas says.

They call from their New York apartment where they’re – happily, as it turns out – sheltering in place. This interview has been edited for length and clarity.

You both write that what makes a marriage last is how you behave when you’re confronted with the really big stuff. This pandemic would certainly qualify. Can you tell me some of the best advice in the book that would help as we all hunker down together?

Donahue: James Carville said, “When you find yourself in an argument that’s going around and around, just kick the can down the road. You don’t have to talk out every little thing.” And that’s the honest-to-God truth. He said that behind every successful marriage is a whole collection of cans.

Thomas: Kyra Sedgwick said, “You have to go into marriage with no Plan B.” If you’re looking for the escape route, there are exit signs everywhere. But to really get the juice out of the orange of a relationship, you need to stick with it.

Marlo, you said that a good marriage is when both people don’t go crazy at the same time.

Everybody goes a little nutty at one time or the other, right? But if you both panic at the same time, you’ll also get into a bigger fight. If one of you stays cool and calm while the other one has their fit, it’s bound to fizz out.

Jamie Lee Curtis said something she learned from recovery is if you stay on the bus, the scenery will change. That’s a good metaphor for relationships.

Thomas: It’s about constancy. It’s about not running away at the sign of trouble.

Donahue: You know, they got married in a cloud of lust like most of us. I’m Catholic, and when I met Marlo, I had impure thoughts! But after that, they had stuff they had to work on. Many couples in the book went to couples therapy: Bryan Cranston and Robin Dearden, Neil Patrick Harris and David Burtka, Ron and Cheryl Howard. Bryan Cranston put it interestingly. He said, “It’s not that we wanted a referee; we wanted an interpreter.” We think we know what our mate is thinking all the time, but we really don’t. And I think therapy is an act of love, really.

What are you learning about each other during lockdown?

Donahue: Well, this has been a fascinating experience for me because I get to watch Marlo all the time. Listen, you’re going to learn more about your significant other during the pandemic than you may have learned throughout the rest of your relationship. I mean, she is a water bug, going five places at one time in all these different directions. It’s never been so clear to me that …

Thomas: What’s clear? That I’m a water bug?

Donahue: Your energy, your purpose, your commitment – you keep on keeping on.

Thomas: I also think having three meals a day together has been very bonding. We never have three meals together except if we’re on vacation.

One of the conclusions you made in your book is there is no one secret to a lasting marriage – that there are a million secrets, and you must keep looking for them.

Thomas: A lot of people put out their best silver and their most fun personality when the company comes over. You can’t just wait until somebody comes over. It’s about being kind and courteous. Phil might say, “You look like you need a pillow” and get me a pillow. He’ll read me some op-ed piece that he really liked because he wants me to feel what he felt as he read it. That makes love grow. That makes lust continue.

Your 40th anniversary is on May 21. How are you celebrating?

Thomas: We’ll make dinner, do the laundry (both laugh).

Donahue: I have to admit that (usually) we have a lovely housekeeper who’s been with us for 30 years, and we do not do the laundry ourselves. Our washer and dryer are stacked, and I didn’t know which was which! They both have a round door.

Thomas: He’s fixated on that.

Donahue: I mean, this has been a real education.