"We kept referring to [Helena] as ‘Chandler's father,' even though Chandler's father was trans," Marta told BBC's World Service's The Conversation. "Pronouns were not yet something that I understood, so we didn't refer to that character as ‘she.' That was a mistake."
The full interview will air July 11.
Marta, who also co-created Netflix's Grace and Frankie with her producing partner David Crane, said her professional settings are drastically different now.
"I like very much to create an environment where we have a happy set and a happy crew," she said. "It's very important to me that where we are is a safe place, a tolerant place, where there's no yelling. I fired a guy on the spot for making a joke about a trans cameraperson. That just can't happen."
Kathleen appeared as Charles Bing, who used the drag name Helena Handbasket, in two episodes during Friends' seventh season in 2001. Charles and Chandler had a strained relationship after Charles split from Chandler's mother Nora, played by Morgan Fairchild.
Kathleen also looks back on her time on Friends through a different lens, telling Andy Cohen on Watch Happens Live in July 2019, "Of course I wouldn't do it now because there would be real people able to do it."
The actress said she was approached about the part by David while performing in a show about legendary performer Tallulah Bankhead.
"He came up to see the show in San Francisco and came backstage and said 'you have to play Chandler's dad,'" Kathleen revealed, "and I thought, 'Okay, a woman playing a man playing a woman. I haven't done that'. So I said ‘yes.'"
As for Marta's admission of failure? It comes on the heels of her concession that Friends severely lacked in the diversity department.
"Admitting and accepting guilt is not easy," Marta told The Los Angeles Times June 29. "It's painful looking at yourself in the mirror. I'm embarrassed that I didn't know better 25 years ago."
In similar fashion, Marta indicated that increased social and political awareness were to thank for understanding the error of her ways.
"It was after what happened to George Floyd that I began to wrestle with my having bought into systemic racism in ways I was never aware of," Marta explained. "That was really the moment that I began to examine the ways I had participated. I knew then I needed to course-correct."
In response, Marta made a $4 million donation to Brandeis University' African and African American studies department.