Viggo Mortensen loves working with director David Cronenberg because his sets are always fun.

Although the movies the pair have worked on together, crime thrillers 'A History of Violence' and 'Eastern Promises' and most recently 'A Dangerous Method' - which details the intense relationship between Carl Jung and Sigmund Freud - are not comedies, Viggo says they always have a laugh on set.

He told Digital Spy: "The best thing an actor can be is flexible, because all directors are different and all actors are different. Some people like to joke around on set and that can be good, because it relaxes you. Other actors are very serious and don't want to talk to anybody. I tend to have fun. I take seriously what I do, but I like to have a good time, which is why I like to work with David Cronenberg."

Viggo believes the tone of the film shouldn't affect the atmosphere on set and says he and the other actors Keira Knightley, Michael Fassbender and Vincent Cassell loved joking around.

He explained: "I had fun with Keira and Michael who I didn't know and Vincent who I'd worked with on 'Eastern Promises' - I knew he liked to keep things light. So, it was a fun set, which was good, because if you start thinking, well, we're making an important historical document, it can become quite dull. There's nothing wrong with having a laugh, if you've done your preparation."