The 27-year-old actress explained: "We had to do one scene 30 times before we got it right. It was mostly my fault. I had to meet her at the lift and walk her through the office, chatting while picking up papers and signing things.
"It required a lot of coordination of dialogue and action, and I kept fluffing it. You’d think a big star like her would get impatient, but instead we were in hysterics."
With the likes of Victoria Beckham, Naomi Campbell and Adele also starring in the US TV series – which drew to a close last year – America admitted she was surprised by how "down to earth" the fashion designer was and claimed Naomi was completely different to her high maintenance reputation.
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She added in an interview with the Daily Mail newspaper: "When they started bringing in guests like Victoria Beckham, Adele and Naomi Campbell, we welcomed them. Victoria was very friendly and down to earth. And Naomi was a surprise.
"Judging by the bad publicity she’s had, you’d think she would be very difficult, but she happily poked fun at her reputation for throwing phones by posing with one in front of the camera."
Now the series has drawn to a close, the actress has taken home her character's eponymous thick-rimmed glasses, frizzy wig and metal teeth brace and a permanent reminder of 'Ugly Betty.'
America – who is currently starring as Roxie Hart in the West End production of 'Chicago' - said: "I’m keeping them safe. I don’t suppose I will ever wear them again, even though there is talk of making an 'Ugly Betty' movie, but it is nice to keep them because we defied the old showbusiness belief that looks are everything by proving that they aren’t."