Growing up in an immigrant Korean family in New York, Keli Lee rarely saw herself reflected on screen. “It was just something that existed and was never questioned because we wanted to assimilate as quickly as possible, so we consumed as much American culture as possible.” It was only when Lee got a job in television that she not only noticed TV’s lack of diversity, she vowed to do something about it.

Beginning her career as a casting assistant on Clint Eastwood’s Unforgiven, she joined ABC in 1991 and became head of casting in 2005. Conversations with producers led her to believe she was not alone in the desire to find new and diverse talent. “Producers were seeing the same faces and didn’t know where the rest of the talent pool was, so in response to that I thought, well if you don’t know where they are, we’re going to go and help find them.”

So in 2001, the ABC Casting Department Talent Showcase was born. Lee and her casting team have auditioned 1,400 actors in LA and New York every year since, identifying the best 14 to 20 who perform at a live show for media executives from across the entire industry. “You’re selecting the actors as an investment and you need to give new talent as many opportunities as possible. The way we did that was to expose them to competing studios and networks, as well as writers, directors and casting directors.”

It would appear that Lee’s “investment” is paying off. If it wasn’t for the 2007 showcase, one Lupita Nyong’o might never have become an Oscar winner. The 12 Years a Slave star is perhaps Lee’s most high-profile find, but others who have found success from the Showcases include Randall Park who stars in the upcoming series Fresh off the Boat, as well as Grey’s Anatomy star Jesse Williams, Chadwick Boseman (42) and Dania Ramirez (Devious Maids).

“What we have is a talent incubator where we find actors at a very early stage of their career and then we nurture, develop, invest in and mentor them. We’re not auditioning on their credits, but on their potential – Lupita certainly had high potential,” says Lee.

Crucially, and unlike some other US network as well as UK initiatives, Lee and her team aren’t hived off in a separate diversity department – their day-to-day jobs involve casting all of ABC’s series from Grey’s Anatomy to Modern Family and Scandal, and if cast lists aren’t diverse enough, they are quick to intervene.

For Lee, Modern Family was a major turning point. “The gay couple made a huge difference across America. The feedback we get is that they may not represent you, but they are you – they are relatable.”

Ironically, the large and diverse cast initially put some actors off auditioning. “Modern Family was not the easiest to cast because it’s such a large ensemble and most actors want to go for a lead part. When it became a huge hit, they were full of regret!”

Although there were personal reasons for addressing the lack of diversity in television, Lee is particularly passionate about senior executives fostering the right working culture.

“It has to come from the top,” she insists, stating that at ABC, the diversity drive came from the very top – from Disney chairman and chief executive, Bob Iger.

ABC’s biggest successes in terms of diversity on-screen, however, are largely down to one woman – Shonda Rhimes. The showrunner will have three of her dramas airing back-to-back on Thursdays in peaktime on ABC from the autumn: Grey’s Anatomy at 8pm, Scandal – which has had three stellar seasons – moving to 9pm, and her new drama, How to Get Away with Murder, airing at 10pm.

Scandal has become a global phenomenon, not least because of its star Kerry Washington, who was a shoo-in for the part, Lee reveals. “Kerry Washington was it, she really was and she really made it her own.”

The series is now credited with giving rise to an increased focus on diversity on-screen across all of the networks, from NBC’s State of Affairs with Alfre Woodard (Hill Street Blues, True Blood, 12 Years a Slave) playing the part of the US president, to CBS’s Stalker, led by British actor Elyes Gabel, who is of Middle Eastern descent. The argument about the “flight of the black British actor” to the US is one Lee is familiar with, not least because it was a complaint put forward by one of ABC’s current stars David Harewood, who recently told Channel 4 News he would be “driving a bus if he’d stayed in the UK”. Although Lee has met Lenny Henry to hear his views on the lack of diversity in UK broadcasting, she refrains from making any direct criticism of the industry.

She does, however, point to the importance of remaining open-minded about every part, even when written for a character with a particular background, as proved by Harewood when he auditioned for ABC’s upcoming Selfie, starring Doctor Who’s Karen Gillan. “The role was written to be a white Jewish guy and David came in and he was brilliant and funny. It was an interesting moment: we wouldn’t have imagined David in the role because of the way it was described, but David came in and was just brilliant.”

In the networks’ autumn season line-ups, ABC leads the way with six new shows reflecting cultural and ethnic diversity, including comedy pilots such as Anthony Anderson’s Black-ish, Eddie Huang’s Fresh Off the Boat and Cristela Alonzo’s solo show Cristela. “Easily the most diverse slate on broadcast television”, according to the Hollywood Reporter.

Launching the schedule to advertisers, British-born Paul Lee, ABC’s entertainment group president, said: “If you look at shows now that lack diversity they feel dated because America doesn’t look that way any more”, adding: “Look, it is a mission statement to reflect America. We think that’s our job.”

There’s a clear business case for diversity, as some 40% of the US population come from ethnic minority backgrounds – the equivalent figure for the UK is 14%, but both are growing rapidly. UCLA’s recent diversity report noted that dramas and comedies with diverse casts excel in the ratings, but also that most shows it examined lacked diversity both on- and off-screen – which, it concluded, “does not bode well for the Hollywood industry as the nation’s population continues to diversify at a dizzying rate”.

Both Paul Lee and Keli Lee point to the importance of finding “authentic stories” told by great storytellers such as Rhimes and Oscar-winning 12 Years a Slave screenwriter John Ridley, rather than engaging in superficial multicultural box-ticking. Ridley has signed an overall deal with ABC, which includes the 11-part series American Crime about the impact of a racially charged incident on the main characters.

Paul Lee believes that diversity behind the camera is crucial to finding the right stories and on-screen talent: “You need the storytellers and you need the executives.” Finding more of those storytellers is the next big challenge, according to Keli Lee. One approach has been to pair writers from diverse backgrounds with actors in the showcase. “The positive is that more people see it as a space they can work in. A television career is not encouraged in ethnic communities.”

Finding such writers is not happening fast enough, she acknowledges, but says: “This is a long term investment and that needs support from leadership. It takes time to change attitudes – you need to change people. My grand vision is that we won’t be having this conversation.”

Lisa Campbell is director of the Guardian Edinburgh International Television Festival. Keli Lee appears on Thursday 21 August on the panel: Minority Report: Is TV Racist?. The festival runs from August 21-23. Tickets and more info at www.geitf.co.uk. The interactive Edinburgh TV Festival app is also available to download now