The 29-year-old actress plays a singer-songwriter in the film and her co-stars were full of anticipation for her first musical scenes, and were left pleasantly surprised, reported the Daily Mirror.
"The tension was thick with anticipation and then she sang and everyone was like, 'Oh my God, she can sing'," Knightley's co-star Mark Ruffalo said.
Knightley recently joked that while she was pleased with her voice in the movie, she wanted to "sound like Adele".
"I had actually done it on one other movie but in a very different style, but no, I've never sung like this. I was actually quite disappointed that I didn't sound like Adele," she previously said.