Kristen Stewart thinks fear is ''life's biggest killer''.

The 'Still Alice' star believes trepidation can be ''motivating'' in smaller quantities but insists it can get ''ugly'' if it starts to control aspects of your life.

She said: ''I think the biggest killer in this life is fear. I think a nice healthy dose of it can be motivating, but if you let it debilitate and dictate how you treat other people, it's very ugly.''

And the 25-year-old actress has admitted she was ''nervous'' ahead of her role in latest film, 'Equals', as it was incredibly emotional to film.

She explained: ''I was nervous going into the movie because of how simple it is and overtly emotional. I'm 25. I've been through some stuff and I knew it was going to be painful and I knew if I didn't do it right I would have hated myself.

''[The hardest part was] not allowing yourself to feel something. I wear my heart on my sleeve. If I'm in a mood, my mood shows.''

During a press conference for the science fiction romance film, Kristen also pondered life's bigger questions

She told Deadline: ''If you don't have passion for something - maybe not for another person - but if you have drive and passion and curiosity, that's what keeps our world spinning.

''If you take that away, why would you want to learn anything? Would we still exist if love didn't? No.''