Transcript for Lady Gaga was 'blown away' by Bradley Cooper's voice for 'A Star Is Born'
Reporter: It's one of the most highly anticipated films of the year. Could I ask you a personal question? Okay. Reporter: A raw and honest love story. Do you write songs or anything? I don't sing my own songs. Reporter: Already garnering critical acclaim and Oscar buzz, Bradley cooper and lady gaga have undeniable chemistry, transcending the silver screen in "A star is born," in theaters today. I think taking off my wigs, taking off my makeup, going back to an earlier place in my life to play this character -- it's an interesting thing for me to reveal. To experience. Reporter: Gaga plays Allie, a waitress whose dreams of making it big have stalled. ??? Maybe it's time to let the old ways die ??? Reporter: But her star ascends when she meets cooper's character, Jackson Maine, a legendary musician battling his own demons. ??? Aren't you tired trying to fill that void ??? ??? or do you need more ??? ??? ain't it hard keeping it so hardcore ??? Is that me? That's you. You just write that now? Yeah. Reporter: The movie marks a career milestone for both actors. For cooper, his directorial debut. For gaga, her first leading role. The kernel of this movie was really just a love story. You know, two people honestly in love with each other and what that means. I did a cancer fund-raiser, I performed that night, Bradley was in the audience. The next day he wanted to meet me. And he came over, and I looked in his eyes, and I just had an instant connection with him. And we were so comfortable together. He made me feel so comfortable. He said to me, do you mind if we sing together? I said, sure. Then he opened his mouth to sing. ?????? I was blown away by his voice. Because he sings from his gut. He sings from his soul. I was overwhelmed with his ability to tell a story with his voice. That's why I wanted to do this because I believed in him so much. Reporter: Cooper believed in her too. It's one thing to be talented, then to harness that talent. I'm just so glad god gave her the talent she has and he chose her. Because that's quite a vessel to go through. Because really what you do with it, and she's such a beautiful human being. Reporter: Their mutual admiration and respect for one another is evident in their performances. Culminating in one pivotal scene. When we sing "Shallows" together. ??? In the shallows ??? For me in that moment all I do is go, oh, I'm on set, an actress. Bradley cooper, even though he's my friend, my dear friend, I was able to go, oh my gosh, this is new, this is real, this is something I've never done before. That's where I drew those circumstances from. I love this song. ??? Tell me something girl are you happy in this modern world ??? ??? or do you need more ??? ??? is there something else you're searching for ??? I don't think that those words could be more relevant to these times right now. It's powerful. It plays a character in the movie. Because then in the chorus she says -- ??? all of the demons watch as I die I'll never leave the ground ??? Watch as I die, I'll never leave the ground. They can't hurt us, we're far from the shallow now. You about made me cry, reciting those lyrics. ??? In the shallow now ??? Reporter: Much like the actors, the characters form an immediate connection. I don't sing my own songs. Why? I just -- I just don't feel comfortable. Why wouldn't you feel comfortable? Well, because like almost every single person that I've come in contact with in the music industry has told me that my nose is too big and that I won't make it. Your nose is beautiful. Are you showing me your nose right now? You don't have to show it to me, I'm looking at it all night. Oh, come on. My initial thought, how are they able to put chemistry and energy in the film to this degree? I've never seen it. He said it in the movie and he said to it me in real life, all you got to do is trust me. All you got to do is trust me. The truth is that I really trusted him. I can't even tell you how much this has changed my life. ??? Maybe it's time to let the old ways die ??? This is his directorial debut, but I feel like he's been a director in many ways his whole life because of his love for movies. I wanted to tell a love story, something that could connect to people. I know that people that I know in my life have experienced love and loss. So that was an ace in the hole in terms of a story people could relate to. You're stripped down, bare for the world to see. Why was that important for your character in this movie? That's just who Allie is. She's a real woman who also has a talent, but who has completely given up on herself. She's so different from me. I really believed in myself when I started out. I was like, I'm going to do this. My friends called me gaga. Like, all right, yeah, I'm gaga. ?????? Reporter: That name, gaga, now synonymous with some of the biggest hits in pop music. "Just dance." "Poker face." ??? Poker face ??? ??? my poker face my poker face ??? Reporter: And "Born this way." ??? I was born this way ??? Reporter: Her songs dominating the charts for the last decade. I hit the ground running. I knocked down every door to play every club in New York City. I mean, I had something to say. I wanted to say it. I believed in myself. But this girl, Allie, she's jaded by the business. She's been told she's not beautiful enough. What's amazing is that this is a love story. And it's his love for her that gives her wings to fly. I was wondering if there's a part of you that goes, I remember those days of singing in places that people would never imagine that I would even sing in before I was this megastar. So did you relate to that part? I did relate to that part. But it was -- it was different. It was different. I really had to go back to an earlier time in my life, more like high school. In high school, you know, I got made fun of for having big dreams. I got made fun of for loving to sing and for loving to be in all wanting to be a musician. During that time I was bullied. I was made fun of. I was depressed. And that is where I drew from, from my life experience, for this character. Reporter: While in real life gaga is nothing like her character, she says Allie has stayed with her. The night after you stopped filming, you went back to the blond hair. Because that was something else different in the movie about you. Yes. People will understand why when they see the film, why I wanted to escape from Allie very quickly. But I was foolish to think that changing my hair color would do that. Watching the film again, I'm right back in there, in that character. She's still very much inside of me. Reporter: For "Nightline," I'm Michael Strahan in New York.
This transcript has been automatically generated and may not be 100% accurate.