The acclaimed film sees Hazel Doupe excel as the outsider looking in, reprising the role she played in Birthistle's first short.
Having grown up in care, Kathleen has just turned 18 and now finds herself back in her late mother's house.
She gets a job and befriends her new neighbour, Joy (Clare Dunne), but things are never right from the off - and become darker by the day.
"The daily challenges that (when) you go into something as a first-time filmmaker, you think, 'It's going to be x, y, and z that I'm going to struggle with or that I won't know how to do' - they're actually not the ones, it's always something else! You go, 'I won't do that next time!' But that's sort of part of the journey, which is great."
Birthistle said that working with British director Ken Loach on the 2004 romantic drama Ae Fond Kiss had been one of the great experiences of her life.
When asked if the iconic filmmaker had given her any advice 20-plus years ago that she brought to Kathleen Is Here, she replied: "Don't eat carbs at lunchtime! He genuinely did [say that]! That stuck with me - that is an important thing to remember! You lose all your energy.
"Do you know what? The whole experience was something that I have brought with me to this point, to the point of being on set and shooting, because not that I'd ever in a million years be anything like Ken, but what is really noticeable and gorgeous and memorable is how he treats people and the general feeling on set, and the experience that hopefully everybody's having, which is just of respect and people liking each other and getting on well.
"He creates this sort of family around him of people who are there for all the right reasons and work really hard, but everybody's just treated the same and really well and really respectful[ly]. Sadly, it's not always the case on set - sometimes it is, but sometimes it's not. It can be a hierarchy, and it can filter down, but Ken and his team just all work together as a very happy family.
"He has a very particular way of working that I wouldn't even dare try to emulate, it's very specifically Ken, but I think it was just important for me that people were happy, that they turned up to work every day! I was so glad that they came back every day as well! (Breathes deeply) 'Everybody's here. Thank God!'
"There's always challenges. It's always hard, obviously, not every day pans out the way you want it to, but I just wanted people to get on and have a nice time."
Kathleen Is Here is in cinemas now.
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