And Killian, who trained at the Drama Centre in London, had to alter his appearance to feel like Tommy.
"I was cast two weeks before we started; there wasn't a wealth of time to research the character," he said.
"They gave me the haircut, I got my earring and the jacket. I started listening to different music and a book I was reading helped me.
"But you just have to put yourself out there. So I would walk around certain parts of town in the character of Tommy, with the accent.
"It is all about confidence; if I can go in to a shop or whatever and relate to people and they to me, then it helps.
"Once you can feel comfortable doing that in real life, you can arrive on set and do it in a high-powered environment.
"I grew up with a lot of it and some of my mates have the accent. I don't naturally have that accent, but you hear it all your life, so you can slip into it.
"It was just a case of refining it. You have to iron out the syllables."
Killian (27) said fans are often disappointed when they meet him in person.
"People expect me to be Tommy, they expect me to have the voice and accent," he said. "They can get attached to the character and sometimes I feel as if I'm disappointing people."
Killian said that, although his haircut helped him to get into character, he has often had worse styles.
"All these things help; a huge part of the process for creating a character comes down to costume, hair, make-up," he said.
"I had this kind of hair when I was 14, an even tighter skinhead cut. I don't know why I thought that was a good look."
The youngest of six children, Killian said his parents have been hugely supportive, if not a bit shocked.
"My parents and extended family are great, but I tell them that they are not obliged to watch it.
"But if they do, I'm not responsible for the trauma they will have experienced; I won't be paying any counsellor bills.
"At the same time, although my parents wouldn't choose to watch it if I wasn't in it, I think they are enjoying it.
"I am sure there is a part of them that would prefer me to be in something like Downton Abbey, but they are sticking with it."