Zac Efron felt he needed to ''step up his game'' after watching Lily Collins act.

The 31-year-old actor stars opposite the 29-year-old actress in 'Extremely Wicked, Shockingly Evil and Vile' and admits he felt he had to bring his A-game after seeing Lily on set.

He said: ''I remember a few days into filming, I just came to work to watch on set because [Lily] was having scenes with Haley Joel Osment, and I was watching her performance and I was blown away. She was bringing it, and this was, like, the fourth of fifth day of filming. I was like, I really need to step up my game. Lily is killing it.''

In the movie, Zac plays serial killer Ted Bundy, who charmed his 30 or more female victims before murdering them in the 1970s, and he hopes to get across Ted's charming nature, which led so many women to be taken in by his lies.

He added to E! News: ''The most exciting thing for me was being able to sort of lend that Ted Bundy charisma to the part. I hope that's what comes across the most ... He was the first mass manipulator of the world. It's kinda crazy [that people] didn't believe [it happened]. People that watched the trial, that were with him in prison, on his defence team, anybody who knew him growing up, they never had bad things to say about the guy. Not a single person. And all the way to the very end of his trial when he was finally found guilty, a lot of people still didn't believe it ... it's kinda crazy. Luckily, I didn't have to get into his mind. I got to observe and figure out what he would be up to or what he would be thinking ... he's so off the map.''