The Hangover starred Bradley Cooper, Zach Galifianakis and Ed Helms and became a huge hit in 2009. Photo / AP

The Hangover was an unexpected hit.

With no big name stars and a small budget, the 2009 comedy went on to make more than $US450 million worldwide, won the Golden Globe for Best Motion Picture — Musical or Comedy and inspired two sequels.

To celebrate the classic movie, we've tracked down some surprising facts about The Hangover.

ALMOST CAST

Paul Rudd and Jack Black both passed on the movie and director Todd Phillips has previously confirmed that Zach Galifianakis wasn't his first choice to play Alan.

"When we were writing, we did have [other actors] in mind," he 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."

Jake Gyllenhaal and Thomas Haden Church were both considered for the role at one stage.

The director also revealed that he spoke to Lindsay Lohan about playing Jade, the role that was eventually played by Heather Graham.

"I did meet with Lindsay Lohan a little bit [before casting Heather Graham], and we talked," he told THR.

"Honestly, it felt like she ended up being too young for what we were talking about. People love to attack her for everything like, 'Ha, she didn't see how great The Hangover was going to be. She turned it down.' She didn't turn it down. She loved the script, actually. It really was an age thing."

BIG BUCKS

Once the movie had been cast, the movie studio was nervous that there weren't any big name actors so said they were going to trim the film's budget and halve director Todd Phillips' $US6.5 million fee.

The director's agent went back to the studio and said Phillips would do the movie without an upfront fee in return for at least a 15 per cent stake in the movie.

They agreed and thanks to the film's success Phillips walked away with nearly $70 million.

UNDER THE RADAR

Las Vegas is one of the biggest tourist destinations in the US, but according to Bradley Cooper no one was really that interested in the fact they were shooting a movie.

"People did not react to us," he told THR.

"That's the one thing about Vegas — they were completely indifferent. We would go in the elevator at five in the morning after shooting, and I had huge scratches on my head, full make-up, and they don't give a f**k. It was unbelievable."

MIKE TYSON WAS HIGH

The boxing champ admitted in a 2012 interview that he was under the influence of drugs when he shot his scenes for the movie.

"(During) the first Hangover, the guys just had my back so much," Tyson told TV host Graham Bensinger.

"They carried me real good. I was a mess. I was overweight ... I was a pig, high on cocaine. They had to know I was messed up. I couldn't talk."

MISSING TOOTH

You might recall that Stu, who is played by Ed Helms, loses a tooth after the messy night in Las Vegas. And it turns out the missing tooth is 100 per cent real.

"I have an implant," Helms explained to People magazine. "An adult tooth never came in and when I was 16, they did a permanent implant.

"We started to do different tests with prosthetics and blacking it out and nothing worked. I wasn't eager to take out my implant because my mouth is healthy, but I talked to my dentist and he was like, 'Yeah we can do it!' My dentist was really into it."

PAY RISE

Bradley Cooper, Zach Galifianakis and Ed Helms got paid less than $1 million each for the first Hangover movie.

But the movie's incredible performance at the box office meant they got a bigger pay packet for the 2011 sequel.

The trio each got paid $10 million for The Hangover Part II and then $15 million each for The Hangover Part III.