Finding the right cast is vital to the success of that project. Could you imagine the first Terminator movie without Linda Hamilton and Arnold Schwarzenegger? At one stage, it was supposed to be O.J. Simpson heading the film. Think about what a disaster that would've been. Well, as hard as casting an action film is, casting a comedy is 100 times more challenging. For instance, the cast of Wedding Crashers made the movie one of the best wedding/buddy films around just as the cast of The Hangover turned a small pitch from screenwriters Jon Lucas and Scott Moore into a mega-franchise worth a fortune.

It's crazy to think that Jonah Hill once turned down a role in the franchise. How did he not have the foresight? Well, luckily, the four actors chosen to bring the movie to life knew better. Thanks to the Hollywood Reporter, we know now how director Todd Phillips (who was later nominated for Joker) cast this hilarious first film.

Going With Lesser-Known Actors Had Its Consequences... At Least In The Short Term

The very first person cast in Todd Phillips' film was The Office star, Ed Helms. He was the perfect choice to play the 'everyman' role. However, Todd had a much, much, much, more challenging time finding the right people to bring the other characters to life. According to The Hollywood Reporter, Todd actually offered two of the other roles to Paul Rudd and Jack Black. However, both of these two legendary actors passed on the movie, for whatever reason. Therefore, Todd decided to spend time looking at lesser-known actors.

But this change of direction had consequences.

The Hangover cast ed helms
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Back in 2009, the studio system was still dead-set on casting major stars in their movies. They were convinced that this was what would guarantee them a decent return on their investment. So once Todd decided to cast virtually unknown actors in The Hangover, the studio trimmed their previously agreed-upon budget. Actually, 'trimming' may be too kind of a word. They also decided to slash Todd's $6.5 million director's fee in half. Todd and his agents later came back to them with an agreement that stated that he would basically forgo his fee altogether in exchange for a HUGE amount on the backend. Essentially, Todd had major faith in the movie. And this faith paid off. According to The Hollywood Reporter, he made something like $70 million in returns from the first Hangover movie... He gambled, and it paid off.

Related: The Truth About Casting 'The West Wing'

Casting The Boys Around Ed Helms

Todd Phillips made some smart moves casting Ed Helms as well as Bradley Cooper, who he was familiar with due to him auditioning for his other work.

"I had heard that Ed Helms had one of the three roles," Bradley Cooper said. "I auditioned for Starsky & Hutch with Vince Vaughn years earlier, and I remember thinking Todd was the coolest guy in the world. Then I sat down [for a meeting], and he was very nice and cool as hell with the sunglasses. So I actually thought there's no way in hell I'd get this role because he's sort of the alpha, really cool guy. But we both love movies. There Will Be Blood was coming out soon, so we exchanged e-mails and went to see There Will Be Blood together at Paramount. And then that was it -- I didn't hear from him. I remember checking in, and they said, 'Yeah, budgetary problems; they're going to need a name.'"

Obviously, Bradley eventually got the role, but for a while there, it truly looked like it wasn't meant to be.

Related: This 'The Hangover' Scene Almost Cost A Stunt Double His Life

As for the role that Zach Galifianakis got, that too took a lot of work. Jake Gyllenhaal, Jonah Hill, and Thomas Haden Church were all considered before Zach.

"When we were writing, we did have [other actors] in mind," Todd Phillips told The Hollywood Reporter. "Quite honestly, we were writing the brother-in-law as a younger brother they had to take along with them -- like a Jonah Hill character instead of Zach. Then we thought it'd be so much more awkward if it was an older brother who's still at home. I've always been a huge fan of Zach, but Zach didn't want to come out and meet with me."

The Hangover cast elevator
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Of course, Zach Galifianakis claims that he did want to meet with Todd, he just didn't want to audition because he felt he was terrible in an audition room. But the studio (Warner Brothers) was deadset on Zach auditioning so they could see him on tape. Zach did just that and that landed him the role, but the studio was still wary of making it with two guys basically nobody knew. Therefore Todd told Warner Brothers to tell him a budget number he COULD make the film for without further hassle. It was a MUCH smaller number. But Todd took it and went ahead and cast precisely who he wanted. And that included Justin Bartha and, of course, Bradley Cooper.

"I'm doing a play in Williamstown, trying to figure out what the f-- I'm going to do with my life," Bradley Cooper explained to The Hollywood Reporter. "And I'm sitting there in the apartment in between matinees, and I get a text [from Todd Phillips]: 'Are we going to f--ing do this?' I was like: 'I haven't heard from you in f--ing four months! Are you serious?' [He was like], 'Yeah, we're going to make The Hangover.'

Next: Here's What Baby 'Carlos' From 'The Hangover' Looks Like Now