ON SET: Judd Apatow, Katherine Heigl, Seth Rogan and Paul Rudd Get Knocked Up, Part 1

He is one of the most creative and funny people - Judd Apatow. He's responsible for bringing America two of the best shows ever on television, Freaks and Geeks and Undeclared; and in movies, the classics Anchorman and The 40 Year Old Virgin.

Now, he's back in the raunchy comedy, Knocked Up, starring Apatow regulars Seth Rogan, Paul Rudd, Leslie Mann (who also happens to be his wife), Jay Baruchel, and Jonah Hill. It also introduces a newcomer to his world of humor - Grey's Anatomy's Katherine Heigl, who stars as a newly-named E! News anchor. This film is a real family affair, not only with Leslie, but with Judd and Leslie's two daughters as well, Maude and Iris.

We had the chance to be on the set of Knocked Up where I asked Judd why he hired his own kids for the movie. He responded simply, "I just thought that whenever you see kids in movies, they always feel scripted and stiff, and if I had my kids there with us, they would act like normal children and do things that kids don't do because my kids do all sorts of weird things. They're really funny and then they're pissed off, and then they're suddenly happy again and there's violent mood swings all day long, and there's a potential for violence and then they just kiss you hard. I thought if I could capture any bit of that it would show people what being a parent is actually about."

But what if it didn't work out? "I was nervous about it because I thought if this doesn't go well it could be a disaster; I've already ended their acting careers. I said, 'Your acting career ends Friday at six o'clock.' But, you know, I thought it could really be a disaster if they refused to do it; they could show up at work and say, 'I want to go home,' and then I'm a lunatic if I don't let them."

The plot of Knocked Up is pretty simple - but just like every other Judd movie, there's a slight adult humorous twist. When Katherine goes out to celebrate her new E! promotion with her sister (Mann) and brother-in-law (Rudd), she meets Seth at a bar. Due to her getting plastered drunk, she goes home and has sex with him; because she's so busy with her new job, she doesn't realize she's missed her period for a couple months. It turns out she got pregnant from that one night stand with Seth, a schlub who lives with 5 of his friends, trying to start a website to look at naked celebrities. Devastated with the news, she tracks Seth down, of course, still not knowing if she's going to keep the baby.

While on the set of the film, we also had the chance to talk to Katherine, Seth, and Paul. You could tell this was a fun set - everyone was having a blast! We arrived at a school in Brentwood, California; a van took us up to the house where they were actually filming. There were kids yelling and running around everywhere because they were shooting a birthday party scene, the birthday of one of Paul and Leslie's kids; it seems like it would be a scene towards the middle of the movie. When we asked which one was having the birthday, Paul and Seth looked at each other and laughed; Seth chimed in with, "Neither of us are really quite sure whose; one of his daughters. I'm trying to pull a sweet move with the lady (Katherine) to re-initiate myself to their world." The both of them went back and forth jabbing at each other and hysterically laughing at each other during our entire interview.

Of course all of them have known each other for years, working on Anchorman and Virgin together, so getting back on set again for Knocked Up was a blast. But for Katherine, she's the newbie of the group. Not known for doing comedy, she jumped at the opportunity to do this film. "I was psyched! I'm a huge 40 Year Old Virgin fan; I'd seen it like three times - in theaters, I didn't even wait for the DVD. When I got the call to audition for the part, I was just really nervous and really excited to meet these people who made one of my favorite movies, and I loved the script immediately. I thought it was super funny, and again it was just my kind of humor. I like really funny movies."

As far as being on set with Seth and Paul, she says "These guys - we sit around all day just thinking of funny things to say and joking around if you go up to set at all, you'll see it's a beautiful place to be filming in, obviously, Brentwood. We kind of just sit out on the lawn in the sun and hang out between takes; it has a very family vibe here. Everybody kind of knows one another and has worked with each other before and they've been really wonderfully accepting of me and bringing me into the fold and it's just been really fun; they're great!"

And Seth, never working with Katherine before Knocked Up, was so impressed with her comedic charm. "She's awesome; she's really, really, really funny. I'm blown away and terrified by her at the same time; we spend half the movie screaming at each other, and she screams a lot better than I do - she could tear me a new one. Her fiancé's on set all the time and I say, 'You get this sh*t at home?' 'Sometimes.'

Judd enjoys working with the same people in many of his projects. "The people from Virgin worked together very well, Seth and Paul and Leslie, so there's a short hand there and everyone understand the process." Bringing in Katherine was a dream come true for the writer and director as well; "They easily fall into what we're doing because everybody is so in sync, so in that sense it's been fun; we're just trying to take advantage of the real relationships people have. So when Seth has four friends in the movie, they are actually played by his four best friends in real life. And when they start talking it just feels very genuine and funny, in the way that they're funny if you hangout with them."

The plot of the movie is a little odd - again, just as Judd would like it. But it was working with Seth again that it really came into fruition. "After Virgin, Seth and I were talking about writing something for him. And all of his ideas were giant science fiction movies. They were very high concept. And I said, 'Seth, you don't need a big concept to be funny and Virgin you're funny just standing there, talking, you just need a situation that's funny because you're in it as opposed to somebody else, like, you get a girl pregnant - and that's just funny because it's you.'"

But it wasn't just Seth that helped out, Paul jumped in with some of his ideas as well; tweaking the initial script is mainly how it happens. But it's all about the constant jokes between the two actors. Paul first gave us a pretty simple answer, "Well, I think it's just that we know each other better so there's already a familiarity with each other's sense of humor, sensibility. But, it can change on any given day; some days, it's really just snappy and you come up with a lot of different things. It doesn't always work at the time, but it is kind of nice and it is easier because we know each other."

Knowing each other way too well could be a help or a hindrance; for these two - it's a help, cause most of their rants will just end up on DVD. "We generally go dirtier than we think they're going to use or than they actually use," Seth says. Paul added, "Seth and I, when we do things together, will get to a really dark place really fast and every once in a while one of the other guys will say something so dark that even the crew flinches. There's no way that's making the movie; that's not even going to make the DVD."

You'll be hearing plenty more from the cast and crew of Knocked Up, which doesn't hit theaters until next August.