Romantic comedies used to be the only funny movies where women were cast as leads.

Actress Sandra Bullock was so exasperated by this that she would ask to read the roles that comedy star Jim Carrey had turned down, just to see if they could be rewritten for her.

Then came The Heat, her 2013 hit with Melissa McCarthy, which showed that buddy comedies starring women could do well too.

Three years later, the 51-year-old is rewriting the rulebook again with the political drama Our Brand Is Crisis, where she got her long-time friend and former co-star George Clooney - the film's producer - to recast the male lead as a woman so she could play the role instead of him.

And there is no reason this gender switch cannot happen more often in male-dominated Tinseltown, say Bullock and Clooney.

In the new film, which opens in Singapore tomorrow, Bullock - Best Actress Oscar for the sports drama The Blind Side (2009) - plays "Calamity" Jane Bodine, an American political consultant enlisted to help save a Bolivian presidential candidate's flagging campaign.

Based on the 2005 documentary of the same name, which focused on a male political strategist working on the 2002 Bolivian presidential election, the original plan was for Clooney - Bullock's co-star in the Oscar-winning space thriller Gravity (2013) - to portray the protagonist.

But Clooney, 54, tells The Straits Times and other press in Los Angeles that Bullock called him one day and said: "How about if I do it and I play the part?"

The actress reveals that this was not the first time she had eyed a part originally intended for a man.

"My quest started before this film, when I was looking at comedies," says the star who, after appearing in hit romantic comedies such as Miss Congeniality (2000) and The Proposal (2009), found herself being offered more of the same.

"I was like, 'Why are the only comedies available for women romantic comedies?' I was so done.

"I yearned for comedies, so I started going, 'Can I look at every script Jim Carrey didn't want to do to see if that can be switched?'

"It started a while ago, but nothing really popped up that I felt was extraordinary."

The game-changer was The Heat, which paired Bullock with comedienne McCarthy in a story about two chalk-and-cheese police officers forced to work together on a case.

It was a runaway success, earning almost US$230 million (S$330.8 million) worldwide - a feat all the more impressive given the rarity of female buddy comedies.

Bullock felt the movie sprang from "the need for women to have a comedy that wasn't centred on getting a man".

"I love men - I just don't always need to be talking about you," she quips.

The experience with Our Brand Is Crisis was equally instructive for her. "I have learnt that you can't worry about getting a no. You have to keep going forward or you would never work again. So I've learnt that it can't hurt to ask," she says, adding that she was glad she spoke up and asked for the lead in Our Brand Is Crisis, which co-stars Billy Bob Thornton as a rival strategist.

"They could have said no, but they didn't," Bullock says of Clooney and producing partner Grant Heslov. "This is something they developed for a long time, so it could have been very precious and I could have changed the tone those guys were looking for."

But Clooney says he and Heslov would never dream of turning down a heavy-hitter like her. "We really couldn't say no," he says with a laugh.

He, Heslov and Bullock also "have a long history together", having become friends before any of them were successful in Hollywood.

"When we first met, we literally couldn't get hired, for the most part," he recalls.

Jokes aside, Oscar winner Clooney (Syriana, 2005) says he can think of no good reason why more leading-man roles cannot be recast in this fashion, especially when the leading woman in question has enough star power to help the film get off the ground.

He and Heslov "develop things because I might want to work on it as an actor, but it doesn't mean it has to be me that does it".

From the minute Bullock suggested the idea, "we looked at it and said: 'This is silly to even think it wouldn't work.' It made total sense, it's just a natural progression. There are a lot of roles that could be switched".

Our Brand Is Crisis was a critical and commercial flop when it opened in the United States last year, but speaking before its release, Clooney and Heslov - who also produced the Oscar-winning political thriller Argo in 2012 - say it is important to them to make films with a socio-political conscience, even though these are not the easiest projects to get financed.

"It's never been easy," says Clooney. "You have to keep the budgets down in these things - you have to work very quickly and you have to get everything done before everybody figures out what you're doing."

Getting a big name in the lead role, however, makes everything a lot more smooth-sailing.

There is no denying Bullock is more bankable than Clooney - each of her films grosses on average US$70 million, compared with US$62 million for his.

"It gets a lot easier when you get Sandy in there," he says. "That's the truth."

• Our Brand Is Crisis opens in Singapore tomorrow.