"Why does rue have to be black, not gonna lie kind ruined the movie"

"I was pumped about the Hunger Games. Until I learned a black girl was playing Rue" "Totally not expecting Thresh to be some big black guy" "Why did the producer make all the good characters black" " … I don't think he will be able to re-enact Cinna's calm temper and quiet personality … " "Omg thought he was white crying omg wtf this movie will suck"

Many of us will have seen the racist tweets of The Hunger Games fandom over the last few days. At the risk of sounding like the veteran with the thousand-yard stare, I wasn't surprised. Some of the comments above were posted last November, after the character posters were revealed. At the time, I tweeted about the obvious and not-so-obvious racism on display – in these people's minds all fictional characters, but especially those set in "non-modern" scenarios, are white. The expectation even extends into genres outside of sci-fi and cult series, and anything that threatens this notion is labelled as "PC gone mad".

I am a woman of colour with a deep – almost unhealthy – love of popular culture. It is a love that is sorely tested in the face of such prejudice when I am told, loudly and with few qualms, that the stories of people who look like me just aren't viable in a specific universe. It is often explicitly stated by my co-fans that I am not – ever – what they picture when they read these books or hear about these movies. The language may be coded: "She's not how I imagined" or, in the case of interracial couple Sam and Mercedes on TV's Glee, slightly more explicit: "They don't look right together, like, they don't … fit." But the message is clear. We get to be supporting characters – the redshirts – or the villains. But heroes? Um, no. That would make things too … ethnic.

I am a huge fan of The Hunger Games and read the trilogy in great greedy gulps. And as several people have pointed out, author Suzanne Collins made it very clear indeed that Rue was dark-skinned (black, as opposed to the more ambiguous description of Katniss as "olive-skinned"), as was her co-tribute from District 11, Thresh. So it comes to this: if the casting of Rue, Thresh and Cinna has left you bewildered and upset, consider two things. One: you may be a racist – congrats! Two: you definitely lack basic reading comprehension. Mazel tov!

Freema Agyeman, who played Martha Jones, Doctor Who's assistant. Photograph: Graeme Robertson

Of course, fandom has a way of getting upset about non-white casting all the time. In recent years, there has been varying levels of dissent around the casting of Martha Jones (Freema Agyeman) as the Doctor's companion, a black Queen Guinevere (Angel Coulby) in BBC's Merlin, and Idris Elba's casting as Heimdall in Kenneth Branagh's Thor. Every so often there's uproar about the (still-yet-to-happen) possibility of a black Bond. And of course there was the infamous #Donald4Spiderman hashtag of 2010. Whenever these issues flare up, I like to think about Elba's comments to upset fanboys after his casting was revealed: "Can a black man play a Nordic character? Hang about, Thor's mythical, right? Thor has a hammer that flies to him when he clicks his fingers. That's OK, but the colour of my skin is wrong?" Echoed by one Tumblr commenter: "Dragons, sure. But black people? Sir, you go too far!"

Meanwhile, the whitewashing of characters rarely gets more than a cursory glance. That's changing slowly: there was a rightful fuss over the casting in M Night Shyamalan's 2008 film, The Last Airbender, in which three of the four leads were cast as white, even though the source material has them as East Asian or Native American. Last year, George Takei spoke out against the casting of the new Akira film, in which Warner Bros has cast white actors as the two leads (they apparently retain their Japanese names).

I recall reading Enid Blyton's boarding school books as a child and knowing that none of the girls in those books looked anything like me. Years later, I went to boarding school in Nigeria and enjoyed many a midnight feast and eluded many a strict teacher, just as the Malory Towers girls did. I don't recall ever looking around my dormitory of young brown-faced girls and thinking that we weren't "right". We didn't look like Darrell, Sally, Mavis or Gwendolyn. But that's okay. We were real too.

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