Joaquin Phoenix shines in Inherent Vice

Joaquin Phoenix in Inherent Vice (Warner Bros)

Joaquin Phoenix enters a room at the Los Angeles Athletics Club, eager to know if his Inherent Vice co-star Owen Wilson has been badmouthing him. What follows is a vivid tale of how war escalated on-set thanks to Wilson's "super-professionalism" clashing with Phoenix's inability to remember his lines.

"I kept struggling with this one line and he glared at me and said, 'I know karate.' I flubbed the line again and there was this two-by-four from construction and he cracked it clear in half."

The air of bafflement that shadows Phoenix is only amplified in Inherent Vice, a crime comedy/drama set in 1970s California.

The Master director Paul Thomas Anderson adapted the screenplay from Thomas Pynchon's 2009 book. Phoenix plays Larry "Doc" Sportello, a stoner private investigator who sets out to help his ex-girlfriend track down her missing lover, a property magnate played by Eric Roberts.

The story's ambiguousness and complexity has sparked cinema walk-outs internationally but Anderson says confusion is part of experiencing Pynchon's work.

Phoenix lost count of the number of times he was on-set thinking, "What the f*** am I doing? It was almost every day," he admits. "Paul was taking dialogue from one character and giving it to another character, so there were parts where I was genuinely confused.

"He stirred up this feeling of uncertainty in me - not knowing where I stood."

Phoenix admits that despite acting since the age of 8, three Oscar nods and a Golden Globe nomination for his portrayal of Doc, he still takes time to build up his confidence on-set.

"The first couple of weeks you're so nervous, trying to figure it out and everything seems so mysterious. Then at some point it starts to happen. I guess you stop being so nervous and sweating and you become comfortable making mistakes - and ultimately end up having a good time."

Casting Phoenix as Doc was a natural choice for Anderson. "Doc Sportello is a lot like the real Joaquin Phoenix. He's sweet, lovable, romantic and has this great thing where he's intelligent but shocked at how bad things can get."

Phoenix recently marked his 40th birthday, a number he says means nothing - then changes his mind. "Of course it does because everyone that you know will say that. And you go, 'Oh my God, am I supposed to feel differently about things?'

• Inherent Vice is screening as part of the New Zealand International Film Festival.