The 52-year-old filmmaker is taking on directorial duties for the DC Extended Universe movie which is currently in early development stages, and says he couldn't turn his attention away from how "intense" the superhero's life is in the comic book.
He told The Hollywood Reporter: "She came up, and I started getting obsessed with how a young woman could get hardcore enough to need to put on the cowl. Like, what's her damage?
"She didn't have her parents killed in an alley. Who is this person, who decides -- rather than being forced to by their childhood trauma -- decides to pick up this life?
"How intense and driven that person is ... I just couldn't stop thinking about it."
Whedon previously said he wants to develop the character before he starts thinking of people he could cast in the role.
He said: "I don't have my eye on anybody. I feel like I'm creating this character, I'm gonna dialogue with her and then we'll see who joins that later on."
And the 'Avengers' director admitted he "doubts" he will cast a big name in the role, as he wants audiences to focus on the character, rather than the name attached to her role.
He explained: "I doubt it will be a name. This is somewhere where you go and find Batgirl and then you cast her. I'm not against movie stars, they're great, but you need somebody who's going to be just right. And a situation like this, the name carries a lot of weight so its not as critical."
Meanwhile, fellow director Patty Jenkins - who directed the upcoming DC Comics movie 'Wonder Woman' - recently said she was "excited" to see what Joss would bring to the female superhero movie as she enjoys his "fun approach" to making movies.
She said: "If it is done, then I'm excited about it.
"I think it's super exciting. The tone of Joss' work is great for female superheroes. He takes such a fun approach and I think he'll have fun in the DC universe, which will be excited to have him."
Previously, it was claimed the 'Batgirl' project only came into fruition in February, following a meeting between Whedon and those overseeing the standalone picture; Jon Berg, Geoff Johns and Toby Emmerich, President and Chief Content officer of Warner Bros. Pictures Group.
Batgirl has been the alter ego of several different characters in the DC Comics but the definitive version is Barbara Gordon who first appeared in comic book pages back in 1967. She is the daughter of Gotham City police commissioner James Gordon and ally of Batman.