David Foster Adorably Answers Questions for Katharine McPhee While She's on Vocal Rest (Exclusive)

When it came to the biggest breakthrough in her career, McPhee's fiance stepped in to answer for her!

Their romance may have taken the world by surprise, but ET put David Foster to the test to find out just how well he knows his fiancée, Katharine McPhee -- and the musician aced it!

With McPhee currently on vocal rest, she was unable to answer questions while hitting the red carpet with Foster at the Breakthrough Prize gala on Sunday, so Foster obliged by responding for her.

“We’re going to see how you guys are as a couple -- you can answer for her,” suggested ET’s Keltie Knight, before Foster agreed to the challenge.

While the GRAMMY-winning producer needed to do some lip reading to name the designer of McPhee’s gorgeous white dress, he then remembered how he actually bought her the outfit because he loved it so much and “love her in it.”

But it was when the 34-year-old American Idol alum and Waitress actress was asked about the breakthrough moment in her career that Foster, 69, appeared to nail it, judging by her reaction.

As Foster stood behind her while hilariously providing her voice, McPhee -- who was slated to tour this month but postponed due to suffering extreme vocal fatigue -- started mouthing along to his words.

“My breakthrough moment really was my mom teaching me to sing all those years ago, but many people would think it’s American Idol because that’s where I really first gained my notoriety and since then I’ve been on two TV series,” Foster said on McPhee's behalf before the pair burst into laughter.

Reflecting on his own breakthrough moment, Foster named meeting artists like Earth, Wind and Fire and creating music for The Bodyguard, before McPhee silently ran her fingers through his hair and prompted him to name meeting her as another significant milestone. “Great breakthrough moment!” he said on cue.

Having become engaged over the summer, Foster also looked to McPhee when it came to an update on their wedding, before sharing that the two are on a “slow, but great plan.”

In the meantime, they were out to support the Breakthrough Prize event, where the world’s top scientists are honored for their work in life sciences, fundamental physics and mathematics.

“They’re the ones who are out there pioneering the new frontier and we don’t think about it,” Foster said. “We’re just doing our music and our acting while they’re going, ‘OK, we’re thinking 1,000 years ahead.’ I don’t think about that stuff. I think about my next chord!”

On Thursday, the couple stepped out to support another event, the annual Friends of the Israel Defense Forces Western Region Gala, which coincided with Foster’s 69th birthday.

Presented with a three-tiered cake on stage, the music producer stood back as the crowd sang “Happy Birthday,” but couldn’t help hurrying them along. ‘Pick it up a little!” he quipped before returning to his table at the event, which was attended by the likes of Ashton Kutcher, Gerard Butler and Fran Drescher and raised $60 million to support soldiers and families of fallen soldiers in the Israel Defense Forces.

Clearly in celebration mode, Foster was up out of his seat for a surprise musical performance from Pharrell, and he and McPhee could also be seen singing along and dancing when Justin Timberlake’s “Can’t Stop the Feeling” came on.

David Foster celebrates his 69th birthday with Katharine McPhee at the Friends of the Israel Defense Forces Western Region Gala in Los Angeles on Nov. 1. - Photo: Alexi Rosenfeld / Friends of the Israel Defense Forces

See more on the fun couple and their recent engagement below.