“Supergirl” executive producer Andrew Kreisberg admitted that they didn’t expect that fans would respond very positively to Katie McGrath’s Lena Luthor.

In a featurette teasing the upcoming Season 3 of the CW series, Kreisberg said that the “biggest surprise” for them last season was the viewers’ reaction to McGrath’s character. “I think probably the biggest surprise for us last season … [was] just watching the audience’s response to Katie McGrath as Lena. It’s interesting because you have written three or four scripts with the character before you cast them, so you have sort of have an idea of what they’re gonna be, but Katie came in and so made Lena her own and turned her into her own iconic character before our eyes.”

“Watching her friendship with Kara (Melissa Benoist) grow is actually going to be one of the centerpieces of the [upcoming] season,” Kreisberg continued, “so we were really excited that it worked out that Katie came in to our world.”

McGrath joined the series as a recurring cast member last season. The Irish actress was then promoted to a series regular for Season 3, two months before the show ended its sophomore run.

McGrath told Collider last May that she never expected to be promoted to a series regular. “When I signed on to the show, I signed on for only a few episodes,” McGrath shared. “I had no idea what I was going to become a part of. I had no idea that this woman was going to be created. To be a part of that, and to have it happen in an organic way, as they saw what worked and the producers responded to what the fans liked, is a very humbling experience.”

“It’s very rare that you get to be with a character at the start of something,” the actress continued. “A lot of characters are known quantities. Even Supergirl, to a certain extent, is someone that you know because there are so many comic books and so many backstories with her. [But] Lena is a little bit different. She’s kind of the unknown Luthor. You know [her brother] Lex so well. You’ve seen him in so many different incarnations, whereas Lena is an enigma. To be able to be a part of that and to have it become something now that I never expected, is exciting. I never expected to be asked to be a series regular on a show like Supergirl. It’s unbelievable!”

McGrath also revealed that she accepted the promotion because she was amazed by how the writers have kept her character interesting. “The reason I came back for Season 3 goes back to what I said earlier [in Season 2],” the actress told TV Guide. “The writers have so eloquently sewed all these little hints about Lena without actually delivering on a few of them, and that’s really compelling for me. … I’ve got hints of where they want to go but I’m still completely in the dark in a way. … That’s why I want to come back for another season, to deliver on everything that’s been promised.”

“Supergirl” Season 3 premieres on Monday, Oct. 9, at 8 p.m. EDT on The CW.