Jennifer Hudson wasn't sure how to approach her vocals in 'Respect'.

The 39-year-old actress and singer plays Aretha Franklin in the upcoming biopic and she admitted the "toughest challenge" was deciding whether to sing like herself or the late music icon.

She told the Metro newspaper: "Like, do they expect me to sing like Aretha? Because who can do that?

"In which case, do they want me to sing like me? What is expected of me?

"Miss Franklin said she wanted me to do this so that made me wonder, am I supposed to approach it as just myself or her?"

In the end, Hudson decided to focus on the project as "an actress" rather than thinking too much about herself as a singer or fan of Franklin.

She explained: "It ended up being about how blend the worlds.

"I use elements of me and my artistry but this is not led by Jennifer Hudson the singer or Jennifer Hudson the fan who knows the material.

"In this I am an actress first and then the character sings."

However, Hudson - who was handpicked by Franklin before her death in August 2018 after the struck up a friendship more than a decade earlier - noted she was chosen for a reason.

She explained how her "own life trials" -her mother, brother and nephew were killed in a shooting in 2008 - played into her performance, particularly in a heart wrenching moment where an alcoholic Franklin sees a vision of her deceased mum.

Hudson said: "I feel I used my pain to tell her story and I think that is what she saw, beyond the singing and acting.

"It didn't register with me until I was in the middle of those scenes that I don't think I would have been able to tell her story with so much depth and vulnerability without my own life trials."