Bradley Cooper was "nervous" about appearing in 'The Silver Linings Playbook'.
The 37-year-old actor was apprehensive about taking on the role of a psychiatric patient trying to win back his wife because he thought director David O. Russell - who helmed 'The Fighter' - would be very demanding, but he enjoyed the experience and they are now firm friends.
He said: "I was nervous to work with David because I knew he was going to be so demanding - I've seen all his movies and there's really not one bad performance.
"But it was an incredible experience and he's become a dear friend."
Bradley is particularly enjoying his career at the moment because he is doing his best to challenge himself professionally.
He told Total Film: "Lately I've been doing things that really scare me. I've taken on projects that are compelling and challenging."
One such challenge was on 'The Words' - in which he plays a struggling writer who obtains overnight success after stealing another author's work - which was directed by his old friends Brian Klugman and Lee Sternthal and also stars Jeremy Irons, Dennis Quaid, Ben Barnes and Zoe Saldana.
He said: "[I enjoyed it but] there were some very extreme conditions. It's a very complicated story and there wasn't much money or time - it was crazy."