Before "Girls," Allison Williams was best known for two things: singing the "Mad Men" theme song with the lyrics to Nat King Cole's "Nature Boy," and being NBC news anchor Brian Williams' daughter.

"It's incredibly great," the face of "Nightly News" told Vulture about Allison's starring role on the polarizing HBO series (Sundays at 10:30 p.m. ET). Yes, even the bracing sex scenes, which included a moment in this past Sunday night's episode that saw Allison's character, Marnie, head into a public restroom to masturbate. Williams said he felt "unmitigated joy" watching Marnie's "awkward sex scenes." But how does the actress feel about those moments?

"A scene like that just gets added to the pile and gets smushed in," she told HuffPost TV when asked about her character's most shocking moment yet.

Starring Williams, Jemima Kirke, Zosia Mamet and show creator, writer, producer and director, Lena Dunham, "Girls" focuses on four women in their early 20s, living in New York. "That's my life," Williams said about the premise. "It's so grounded in reality that it almost feels like a real show. But we are acting, which is an important detail."

Williams also opened up to HuffPost TV about what she thinks was just as difficult as filming sex scenes, her first scene with Dunham (in bed), whether she's similar to Marnie, and much more.

How did you become a part of "Girls"?
It has a very odd beginning. I graduated from college in the spring of 2010. I made a video called the "Mad Men Theme Song With a Twist." When I moved out to Los Angeles to start acting and become an actress -- which I wasn't allowed to do until I graduated -- I uploaded the video to YouTube, and I heard within days from Judd Apatow through my agents. "We're doing this HBO series and Allison has some vibe that we've been looking for. Can you make sure she comes and reads for it?" So, I got sent the script and I got sent "Tiny Furniture" [Lena Dunham's 2010 breakout indie film] which -- somehow -- I hadn't seen yet. I was probably in that college vortex of pop culture. I watched "Tiny Furniture" and I don't think I blinked. It felt like she was being honest with me specifically. Then I read the script and I thought, "My God, this is so crazy. I feel like she's been living in my hallway and listening to all my thoughts and she's put them into the character." Obviously I put a tremendous amount of pressure on myself. "You gotta get this part. This will be the one that got away if you don't get it."

You and Lena have great onscreen chemistry. Did you get close in real life?
Immediately. She's one of those people you feel immediately that you've known for years. I have to give her a lot of credit: She put a lot of effort into forging our friendship. I was very intimidated by her intelligence and stature on the show. She was like, "Don't worry about any of that. We're just a bunch of girls in the world. You and I are going to be fast friends." It was true and it happened organically. The first scene we shot was when we're waking up in bed together and they're panning up and I have that beautiful mouth guard in. It's very humbling to have that be your first shot on television ever.

The show is not without shocking scenes, and while Lena has given her character a lot of the onscreen nudity, Marnie's masturbation scene felt incredibly intimate. How challenging was that to do?
I feel like it's an important scene. It's one of many for Marnie. It's funny because there are moments where -- yes, that was challenging, but there are other things that are so hard. Sometimes fake laughing is hard once you've done a scene 18 times. I don't want to brag, but I have a reputation for being very, very good at that. It's funny finding what's challenging about acting as you go. A scene like [the masturbation scene] just gets added to the pile and gets smushed in. It's been really interesting being interviewed and being asked about it so much, because I'm like, "Oh, yeah, well there have been seven episodes since that scene." I also feel like we are showing these very intimate, private and specific moments in our characters' lives and it just sort of is a part of Marnie's puzzle.

"Girls" does have a voyeuristic quality to it.
I think the show is real on many levels. In real life, we do things out of character, constantly. A couple of days ago, my shoes were hurting, so I walked barefoot through New York. Someone who has known me my whole life would think that was so out of character. But I did it because of the circumstances. We have so much freedom -- and I have give a lot of this credit to HBO, because they allow us the freedom to explore the different corners of our characters throughout the series. Wait until you see the next episodes to come. Marnie takes deliberate steps out of her comfort zone to see how it feels. It's so nice to do that. As an actor, it's such a great challenge because you get a handle on this person and then someone is like, "This person is going to become another person, and see how she does it." That's so enjoyable. I think girls will be able to relate to that especially, because when our friends do things that are out of character, it's really confusing and almost impossible to advise them through it.

The problem with everything feeling real is that people might automatically assume you're exactly like you're character.
Which means we're doing a good job.

Right, but does it concern you that people might start typecasting you at such a young age?
I think the beauty of acting -- and the beauty of YouTube -- is that if I felt that was becoming a problem, I could make something. Play someone totally different. The crazy thing about that is that I've never played anyone this similar to me in my life. My whole background is character acting: weird costumes, fat suits, playing men, playing animals -- I've never played anyone with whom there's any overlapping Venn diagram. That's been a crazy challenge for me. There have been so many moments where I've been, "Well, I wouldn't do this, but Marnie would; I would do this and Marnie wouldn't." You're sliding levers constantly, trying to adjust and see where the lines meet. It's been great to explore that and it's been a challenge and a pleasure.

"Girls" airs Sundays at 10:30 p.m. ET on HBO

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