Castro noted Hudson is almost unrecognizable -- except for her big, beautiful voice.
Hudson lost an estimated 80 pounds after joining Weight Watchers.
She told Castro, "Like, I was just watching TV last night, and I was like, 'Oh that's Jennifer Hudson. Oh duh!' That would be me!"
Castro pointed out how many women struggle with weight after pregnancy.
Hudson said, "Towards the end of my pregnancy I was like, 'I just want my body back, whatever that body was before, I don't care, I just want it back.' And then I realized I want to do something different with it, because you realize you have that power again, you get yourself back."
Hudson credits her metamorphosis to her most important role -- as mom to 17-month-old David Otunga, Jr.
Castro said, "I'm a mother of two, and I say all the cliches are true, that your life totally changes."
Hudson agreed, saying, "He's my motivation for everything and inspiration, you know? He's like a sponge right now and he's absorbing everything that we do."
Castro said, "Your fiance told me that he sleeps through the night?"
Hudson said, "He has always slept through the night, so we lucked out big time."
Castro said, "We're going to knock on this paper because we don't have wood."
"Girl, knock hard!" Hudson joked.
But Hudson hasn't needed to rely on luck with her powerful pipes. In just a few short years, she quickly catapulted from "American Idol" cast-off to Oscar and Grammy-winner for her role in "Dreamgirls."
Castro recently caught up with Hudson in her hometown of Chicago for a very intimate performance with her sister Julia, fiance David, and baby boy watching from the wings.
Castro said, "Chicago holds a very special place in your heart."
Hudson responded, "We just moved back home and it's amazing, I didn't expect to come back to Chicago so soon, but just bringing my baby home, and we wanted to be around family and him to grow up around family. Both David and I are from Chicago, so we brought him back home to that, because we're both super family-oriented."
The bond that brought Hudson home also compelled her to create the Jennifer Hudson Foundation, helping families affected by violent crime. In Chicago, Hudson distributes toys to needy children in honor of her late nephew, Julian King, murdered along with Hudson's mother and brother on one horrific day in 2008.
Speaking about her nephew, she said, "He was a very giving child, and he got tons of Christmas presents and he would give a lot of them away to other kids, so just to remember him in a very positive way. I just want to make sure we remember him, and it's a healing process for all of us."
While her roots are firmly planted in Chicago, Hudson's career has taken her from Hollywood to South Africa, where she spent the summer portraying Winnie Mandela, former wife of Nelson Mandela in the upcoming biopic "Winnie."
Hudson has also been busy in the recording studio, following up her Grammy-winning debut with a new album due out in February.
Castro asked, "Is it a huge departure from your last album?"
"I feel like it's an evolution from my last project," Hudson said. "Everything is new in a way. I've matured in so many different ways."
She continued, "I got to work with Alicia Keys, and I remember sitting down with her and her explaining to me the material that she wrote for me and created for me, you know -- 'You're an inspiration, and you inspire people, and no matter what you're going through, I hope to uplift people through it.' I'm like, 'Wow, people are watching Jennifer, so let me give you something to look at, let me give you something to listen to, let me tell my story."'