The 40-year-old actor plays drug-fuelled detective Larry 'Doc' Sportello in the romantic comedy drama and director Paul Thomas Anderson - who is up for an Adapted Screenplay Oscar for the movie - joked the star had a great time getting into character during filming.
He said: "Joaquin's a committed actor - you know what he's like.
"This wasn't something that demanded intense commitment, like being in a wheelchair for six months.
"It required him to not wear shoes for the length of the shoot.
"But his f***ing feet were disgusting!"
The motion picture is based on the detective novel of the same name by Thomas Pynchon but Joaquin stopped reading the book second time round because he didn't want to get "confused" by it.
Speaking to Metro newspaper, he said: "I thought, 'I don't want to know this too well. I want to be confused by what's going on.'
"Paul would stir it up and make me feel really confused. He would be like, 'I don't know, man, you're just going to have to figure it out.' "
Paul, 44, added: "We were always staring at the book, trying to make sense of it."