Farwell heard about the opportunity to take part in the film from the charity she turned to when she was diagnosed, Tourettes Action.
Tourette’s is characterised by sudden, involuntary and repetitive movements or sounds, known as tics.
Farwell was diagnosed after experiencing a “tic attack” which led to her being put in hospital and sedated.
Speaking to BBC Radio Solent Dorset on Thursday, Farwell said: “It was punching myself, it was banging my head into walls, it was really, really violent and horrible.”
The aspiring performer, who played a child with Tourette’s in I Swear, said it was a “relief” to be diagnosed, having experienced tics since the age of seven.
She has Coprolalia – like Davidson – and also has Echolalia, which means she repeats what people say, often in a mimicking tone.