The Crouching Tiger, Hidden Dragon filmmaker's latest offering is a 3D adaptation of Yann Martel's fantasy novel about an Indian boy who is stranded at sea with a Bengal tiger for company.
At the film's London premiere, Lee said: "Cinematically speaking, it was the most difficult movie I ever made. Water, kid, tiger, a new language I was not familiar with - 3D - and it does have a tricky ending that sort of pulls the rug under the audience's feet. That was quite scary."
Lee - who cast newcomer Suraj Sharma to play the title role - said he fell in love with the Booker Prize-winning novel's message.
"The book is about the power of storytelling, faith and spirituality, how that is important in our lives," Lee said. "It's incredible, incredible material. I didn't know it was movie material but it really grabbed my heart."
Sharma, 19, landed the role after accompanying his actor brother to the audition.
The student, who joined Lee and co-star Rafe Spall at the Empire cinema in London's Leicester Square, admitted he was unsure about taking on the starring role with no previous acting experience.
"The entire crew was so welcoming and friendly. I went in thinking 'Oh my God, they're going to think this new kid, he's gonna mess up' and I thought they would be a little bit sceptical about the whole thing but they have been so supportive like a family. I never felt scared," he said.
Sharma said his life had changed "a lot" since appearing in the film. "I can't explain how much. I was a normal kid. I'm an ordinary kid in an extraordinary situation... Everything has changed, my outlook, the way I think, everything around me, it's all changed."
Life Of Pi is in cinemas on December 20.