After 40 years of marriage, Marlo Thomas and Phil Donahue are exploring the secrets to a successful union with the help of some Hollywood pals.

The “That Girl” actress, 82, and talk show host, 84, have released a new book titled “What Makes a Marriage Last: 40 Celebrated Couples Share with Us the Secrets to a Happy Life.”

The pair previously spent nine months interviewing celebrity couples whose relationships they’ve admired over the years, including Kevin Bacon and Kyra Sedgwick, Chip and Joanna Gaines, as well as Ron and Cheryl Howard--just to name a few. Their goal was to better understand what works, and what doesn’t.

Thomas and Donahue spoke to Fox News via email about their first-ever project together, the time they were separated and how they continue to keep the romantic spark alive.

NATALIE WOOD’S DAUGHTER CALLS ROBERT WAGNER ‘COURAGEOUS’ FOR SPEAKING OUT IN DOC: ‘I WAS JUST SO PROUD OF HIM’

FORMER BOND GIRL BRITT EKLAND SAYS EX-HUSBAND PETER SELLERS WAS ‘A VERY TORMENTED SOUL’ IN NEW DOC

Fox News: What compelled you to finally sit down and write “What Makes a Marriage Last” together?
Marlo Thomas and Phil Donahue: Last year, we had begun talking about doing something special for our 40th anniversary, when out of the blue, a long-married couple we knew told us they were splitting up. We were shocked — they were good friends and everyone’s favorite couple — and it got all of us talking about our own marriages. Could this happen to us, we thought?  Where did they go wrong? Or for that matter, where did we go right?

Those conversations prompted us to wonder if there really was a secret sauce to a long and enduring marriage — and that made us want to find out. So we decided that a book about marriage — one in which we’d interview 40 long-married, celebrated couples we admire — might be a way to find the answer. And the timing felt right.
 
[And] we’re living in a very negative time — a time when people are lashing out more than they’re reaching out. So we thought a book like this would serve as an important reminder that we're often at our best, our strongest when we're holding the hand — and have the back — of someone we care about.

MEGHAN MARKLE, PRINCE HARRY TELL ALL IN UPCOMING BOMBSHELL BOOK ‘FINDING FREEDOM’: REPORT

‘M*A*S*H’ STAR LORETTA SWIT SAYS SHE’S USING HER TIME IN QUARANTINE TO HONOR HEROES: ‘THEY ARE OUR LIFELINE’

Fox News: You two have rarely spoken publicly about your relationship. Why?
Thomas and Donahue: Because we’re both in show business, and when you’re in show business, your private life is anything but private. The way we saw it, we were very lucky that our paths crossed when they did, and that we’d made the decision to spend the rest of our days together. That was — and remains — sacred ground to us, and we wanted to protect it. This book is actually our first project together, ever. Talk about putting your marriage to the test! And to our surprise, we wound up talking about our own relationship a lot as we interviewed these other couples about theirs.

Fox News: What would you now say has been the secret behind your lasting marriage?
Thomas and Donahue: As we say in the book, the secret to any successful marriage is that there is no single secret. There are a million of them. That’s why the book turned out better than we ever imagined it would.

Each and every one of the couples we interviewed possesses their own unique narrative and have forged their own unique path to happiness. As for our secret, you’ll get pretty good glimpses of it in the book — but one thing for sure is that, once we decided to commit to each other, we also decided that no matter how tough things might get, we would never look for an escape route.

‘BATMAN’ STAR BURT WARD SAYS WAKING UP ‘EVERY SINGLE DAY IN THE MOST WONDERFUL COUNTRY’ IS 'A GIFT’

Phil Donohue and Marlo Thomas pose for a photograph at Gloria Steinem's 50th birthday celebration on May 23, 1984, in New York City. 

Phil Donohue and Marlo Thomas pose for a photograph at Gloria Steinem's 50th birthday celebration on May 23, 1984, in New York City.  (Photo by Yvonne Hemsey/Getty Images)

BEACH BOYS’ MIKE LOVE SAYS HE WANTED TO ‘GIVE A LITTLE GLIMMER OF HOPE’ WITH ‘THIS TOO SHALL PASS’

Fox News: When did you both realize that the other was The One?
Thomas: I’ll take this one. Phil and I broke up for three months before we got married. We were arguing a lot.  We were working and living in different cities, and all of the compromises we were making to accommodate two full and different lives made things so difficult. And on top of that, Phil had the added pressure of raising his four sons alone. Both of us just finally said, “We can’t do this — not to ourselves and not to each other.”  So we broke up.

Phil started dating and I got back together with an old boyfriend, but I was miserable. All I thought about was Phil.  Then I got a call in the middle of the night, and it was Phil. He said, “I never thought anybody could be this irreplaceable.” We were married 12 weeks later.

‘HAPPY DAYS’ STAR ANSON WILLIAMS RE-RECORDS ‘PUMP YOUR BLOOD’ TO ‘I’M GONNA WASH MY HANDS’ FOR CHILDREN

'THE BIRDS' STAR TIPPI HEDREN REVEALS WHAT HER LIFE IS LIKE IN QUARANTINE: 'MY ONLY ROUTINE IS NO ROUTINE'

Fox News: Which couple featured in your book surprised you the most and why?
Thomas and Donahue: Ray and Anna Romano surprised us. Our interview with them was filled with laughs, of course, but then just as suddenly we were all crying. As it turns out, in 2010, Anna was diagnosed with breast cancer, and while she was coping with the terror of that, Ray had to step up to a whole new role in their marriage — and he did that triumphantly. Their description of that time is both chilling and touching, and we were incredibly moved to hear the husband's side of this frightful experience.

FORMER 'GLADIATOR' STAR CONNIE NIELSEN GETS CANDID ON HER FAVORITE MEMORY FROM SET, FILMING 'SEA FEVER'

CAMILLE KOSTEK REVEALS HER QUARANTINE ROUTINE WITH ROB GRONKOWSKI: ‘WE’VE BEEN LOVING THIS TIME TOGETHER’

Jamie Lee Curtis and Christopher Guest [also] really bowled us over. We knew he’d be hilarious — which he was — but we were stunned when Jamie revealed to us one of the secrets of their marriage. We even had to ask her to repeat what she said because we were sure we heard it wrong. We won’t ruin it for you — you’ll have to read the book to find out!

Fox News: Viola Davis said that marriage doesn’t begin when you walk down the aisle. When did marriage between the two of you really begin?
Thomas and Donahue: We talk a little bit about that in the book. We’re both type-A personalities, so we fought a lot in the first year. In addition to being deeply in love, we were also deeply invested in our work, and it was hard to have our marriage and our careers blend at the same time. It took us about a year to find our groove, and we did it on neutral ground — not in L.A. or in Chicago where we had our separate lives, but in New York, where we built our new life — and our marriage — together.

Fox News: How do you keep the romance alive after all this time?
Thomas and Donahue: There are many different ways — but it’s always about time carved out together. We like to take a weekend and just go off on our own. Or we’ll go out to one of our favorite restaurants in our neighborhood. Or we’ll just stay home, make popcorn and watch a movie. We also like to do something that the other one loves.

'LITTLE HOUSE ON THE PRAIRIE' STAR CHARLOTTE STEWART SAYS SHE DOES THIS EVERY MORNING DURING QUARANTINE

Marlo Thomas and Phil Donahue enjoying a date night in New York City, circa 2015.

Marlo Thomas and Phil Donahue enjoying a date night in New York City, circa 2015. (Photo by Walter McBride/WireImage/Getty)

'WALTONS' STAR MICHAEL LEARNED 'PROUD' OF NURSES, CAREGIVERS AMID CORONAVIRUS PANDEMIC

Thomas: Phil will take me to a comedy club, or I’ll take him to a ball game. There is romance in giving each other the gift of your time and doing something they love more than you do. One of the things we discovered in our interviews for the book is that all of the couples take real delight in each other’s company, and they make it a point to find the time to hang out together.

Fox News: All of these couples featured in your book have high-profile careers, many in Hollywood. After hearing their stories, how do you think they’ve managed to maintain successful relationships, all while being in the public eye?

Thomas and Donahue: These high-profiles careers often demand a lot of sacrifice from both spouses. It begins with powerful respect and belief in your partner’s career. Billy Crystal and Alan Alda struggled for years, bringing in little money as they pursue their chosen work, and in both cases, their wives believed in them so completely that they never asked them to stop and ”get a real job,” but instead they worked as a team, making ends meet until their husbands realized their dreams and their lives became secure.

‘BEWITCHED’ STAR ERIN MURPHY REVEALS HOW QUARANTINE HAS KEPT HER BUSY AT HOME

Phil Donahue and Marlo Thomas circa 1979 in New York City.

Phil Donahue and Marlo Thomas circa 1979 in New York City. (Photo by Sonia Moskowitz/IMAGES/Getty Images)

‘HAPPY DAYS’ STAR MARION ROSS SHARES HOW QUARANTINE HAS CHANGED HER DAILY ROUTINE

Thomas: As Phil once said, “Celebrities aren’t celebrities at home with each other.” All of the couples we talked to described an intimate connection that protected them from the public eye. Mariska Hargitay’s husband, Peter Hermann, recalls a favorite marital moment of his that was nothing more than a peaceful afternoon at home with his wife and kids. He said, “We were all in the same room, the dog was fed, there was sunlight, the door was open, a breeze was coming in, and we were like, Yes.” Famous or not, loving relationships endure because the two people cherish what they have together.

‘CHARLIE’S ANGELS’ STAR CHERYL LADD RECALLS WORKING WITH ‘TERRIFIC’ FARRAH FAWCETT: ‘THAT WAS REWARDING’

HALLMARK STAR JILL WAGNER EXPLAINS WHY SHE GAVE HER DAUGHTER A ‘PATRIOTIC NAME’

Fox News: Marlo, your father Danny Thomas also had a lasting marriage with his wife, Rose Marie. What were some of the biggest lessons about having a happy marriage that you witnessed just by watching your parents? 
Thomas: My parents had a lot of separation in their lives. My dad was often on the road, working in night clubs. And my mom was always flying off to be with him and then flying back to be with us kids. They would take us out of school as much as possible, or on holidays, so we could be together as a family.

It showed me that you can define marriage and a family on your own terms. When Phil and I first married, our work had us living in separate cities for the first five years — both of us traveling back and forth to be together. I don’t think I would have had faith in that kind of life had I not seen my parents do it so successfully.

AL CAPONE’S GRANDNIECE REACTS TO TOM HARDY’S TRANSFORMATION IN NEW TRAILER FOR BIOPIC

(L-R) Comedian Danny Thomas and daughter, actress Marlo Thomas with her husband Phil Donahue.

(L-R) Comedian Danny Thomas and daughter, actress Marlo Thomas with her husband Phil Donahue. (Photo by Time Life Pictures/DMI/The LIFE Picture Collection via Getty Images)

'ONE DAY AT A TIME' STAR JUSTINA MACHADO RECALLS AUDITIONING FOR NORMAN LEAR: 'I WAS TERRIFIED'

Fox News: What’s the best way to handle disagreements?
Thomas and Donahue: You’re talking about the F-word — fighting! — and when it comes to the couples in the book, they all have their own ways of going to the mat. And coming back from it. Amazingly, of the 40 couples we sat down with, only Jimmy and Rosalynn Carter — who have been married for 74 years — have stood by their wedding promise never to go to bed angry. The rest don’t buy into that.

Ben Falcone actually joked with us that he tries to go to bed angry when he’s fighting with his wife, Melissa McCarthy… Ron and Cheryl Howard practice a whole list of fascinating conflict-resolution exercises that they learned from their couples counselor — and yes, after 45 years of marriage, they still see a counselor together, not out of desperation, but to keep the marriage healthy and strong.

Fox News: How does humor play a role in your marriage?
Thomas and Donahue: It softens everything. Face it, marriage can be hard work at times, and successful couples take that work very seriously. But often you have to be able to step back and laugh at the absurdity of some of your fights! Are you really going to put on the boxing gloves because someone left the cap off the toothpaste tube, or didn’t put the remote back on the charger? No, you’re not.

'JEOPARDY' GOAT KEN JENNINGS TALKS FRIENDSHIP WITH ALEX TREBEK, REVEALS FAVORITE FILMS TO WATCH FOR QUARANTINE

Phil Donahue and Marlo Thomas attend old Spring Harbor Laboratory's Double Helix Medals at the American Museum of Natural History on December 1, 2016, in New York City.

Phil Donahue and Marlo Thomas attend old Spring Harbor Laboratory's Double Helix Medals at the American Museum of Natural History on December 1, 2016, in New York City. (Photo by Victor Hugo/Patrick McMullan via Getty Images)

CAMERON DIAZ OPENS UP ABOUT MOTHERHOOD: ‘IT’S THE BEST PART OF MY LIFE’

Fox News: What’s one big piece of advice you would give to a newlywed now?
Thomas and Donahue: To keep at it. As Kyra [Sedgwick] told us, you don’t go into marriage with a Plan B. “No matter what,” she said, “we want to work it out.” We heard that time and again from many couples. Elsa Walsh told us, “It’s so much easier to work on being married, and staying married than it is to work on getting divorced.” And that’s true. It takes a lot of effort to get out of a relationship, but if you put that effort back into the relationship, everything is so much more satisfying. Jamie Lee Curtis actually said it best: “What’s the secret to staying married? Don’t leave.”