The 45-year-old actress - who played Joey Potter on 'Dawson's Creek' - has revealed that she's been inspired by her experience of working on TV shows and movies.
Katie told The Sunday Times newspaper: "I feel my education in clothes and their meaning came out of working in television and movies, working with great costume designers and having it explained to me what it means if a character has a cable-knit sweater over a plain sweater, and how that reveals a part of them".
"I fell in love with thinking like that, and letting the clothes really dictate a certain part of someone's essence".
Katie has her own collection with the French brand APC, and the actress has revealed that she also finds inspiration in everyday life.
The Hollywood star shared: "I really look at other people that have an innate sense of style. And I'm like, 'Oh - that with that! OK, that's cool.' I'm always looking for inspiration".
"I like to mix and match classic pieces. I like vintage a lot. Living in New York City I have to have things that are practical and comfortable. The city influences my style".
"Usually I'm in ballet flats, sneakers, baggy jeans and a T-shirt. I like putting it together in a way that doesn't look like you put it together".
Katie's collection has been inspired by her own experiences, too.
She explained: "Our T-shirts are pretty long, because it drives me crazy when you move and a T-shirt rides up".
"I remember the 90s as being about looking really simple and very natural. That's what I wanted this collection to embody - pieces that are cool but not recognisable and can fit into your existing wardrobe seamlessly".