Backstreet Boys' Brian Littrell and Wife Leighanne on Secret to Their 20-Year Marriage (Exclusive)

Only ET was with the singer and his wife as they celebrated 20 years of marriage.

Backstreet Boys' Brian Littrell and his wife, Leighanne, celebrated a major milestone on Sept. 2 -- their 20th wedding anniversary. ET exclusively sat down with the couple at their Atlanta home for their big wedding anniversary -- as well as their 17-year-old son, Baylee -- and they shared how they've been able to maintain such a strong marriage for two decades.

The couple met on the set of the Backstreet Boys' music video for their 1997 hit, "As Long as You Love Me." Brian remembers every little detail and calls the moment one of his most cherished memories of their entire relationship.

"It was just kind of a smack-in-the-face whirlwind of life-changing events that happened right there when we met," he says. "I had a chance to see Leighanne's headshot on the way there, it was about 4:15 in the morning, I was in a van, headed to the shoot, and I saw her photo at the bottom of the stack and it said 'Leighanne Wallace,' and I was like, 'Wow, I have to remember that name.'"

"We ran into each other at that perfect time," he continues. "Neither one of us were looking for love. We didn't really need it at that time, we were focused on our careers and that's just kind of how life happens. It is a blessing that we were opened to this joint venture."

The two recall Leighanne actually turning Brian down when he asked her out for a drink on the night they met because she already had plans.

"She shot me down, and then she goes, 'I am free for tomorrow night,' and I said, 'Well, I did not ask you for tomorrow night, I asked you for tonight,'" Brian says. "I was trying to play hard to get."

Leighanne shoots back, "Well, the question was, 'Did you want to go out tomorrow night?' It was mutual, but again, it was surprising."

Of course, the two eventually started dating and got married in 2000. Brian says the secret to their relationship is compromise.

"You know, a relationship is a work in progress, especially a marriage, and you have to work together," he notes. "You have to be a team. You have to access things in life as a team, it can't be a one-sided thing. Some of the best marital advice I got was from our pastor who married us. ... He said marriage is not 50/50, it's 100 and 100, which, I really like that."

"To be honest with you, I think we are constantly changing," he adds. "I think we are constantly growing, constantly evolving, trying to better ourselves and better our relationship and our family, better our surroundings."

Leighanne stresses that they consciously strive to change together and not apart.

"I think that is one thing that we have been working on because marriages fail because you change separately, and maybe because of your kids and your career -- things going on -- it's easier to separate," she notes. "So if you stay together -- which we always have and try to grow together -- I think that is huge."

"It's just communication," she adds. "It all goes back to that and to really just be real."

ET was actually with Brian and Leighanne for their wedding vow renewals in 2010. The couple watched the priceless footage alongside Baylee -- himself a country singer, following in his father's musical footsteps -- who says he does remember the special day despite being only six years old at the time.

"I remember one thing that was very significant about the whole party was it was my bedtime and I was not having it," Baylee recalls. "I just did not want to go to bed. You guys were like, 'All right, come on.' Like, what was it, 10 o'clock back then? You're like, 'It's past 10, you go to bed.' And I'm like, 'I want to have fun.' I'm like, 'You guys are gonna have fun without me.' You know, I just remember going to bed."

Meanwhile, Brian says their marriage is even better 10 years later.

"I think asking us 10 years ago do we value each other more 10 years ago than we do today, I think we value each other much more today after 20 years, and it will continue, I hope as long as you will have me," he adorably tells his wife.

Still, there has also been some tough times in their marriage. One moment they'll never forget is Leighanne canceling her scheduled Sept. 11, 2001 flight from Boston to LAX at the last minute because she had a bad feeling. The flight ended up crashing into the World Trade Center.

Brian comments of the miracle moment, "I think we have lived many lives together on this journey. We are just blessed to have each other and I think I am blessed to have you in every way. Don't get me crying."

As for what the two have learned about one another over the years, LeighAnne jokes that her husband is a hoarder.

"He cannot keep a clean closet," she shares. "That's the bad thing I know about you. The good thing is that your heart just gets bigger and bigger and I had no idea that it could get so big and be so sweet."

Meanwhile, Brian marvels at his wife's strength.

"You know, when we first met, I knew that she was a strong woman, but I just didn't know the depth of how strong she really is," he shares. "I mean, I know she brings the best out of me. I say that constantly, and I hope that I can just bring a little greatness out of her 'cause she just kind of exudes it. But I just didn't really understand the depth of her strength. She's an amazing woman."