The Australian actor slimmed down to play Valjean in director Tom Hooper's project, and Jackman also shunned drinks ahead of his first day on set to give himself a haggard look.
He tells Britain's The Sun newspaper, "I lost probably 20 pounds to begin with. For me it was about losing as much fat and being as lean as possible. Tom said, 'If you are recognisable, I want people to think you are sick'.
"So for the first day of shooting I didn't drink water or any liquids for 36 hours, which a trainer at the time said is a bodybuilder trick which can give you sunken cheeks and sunken eyes. So I went for it - and slightly regretted it 20 hours in.
"When I see the first shot I am really glad I did it. But I had massive headaches, was slightly dizzy and very, very grumpy."
The diet wasn't the only tough part of the shoot for Jackman - he relocated to England to film the movie, but desperately missed his wife and children.
He adds, "I was living on my own for the first time without my family. Nights were spent protecting my voice, sleeping and generally being very un-Hollywood. I was doing jigsaw puzzles, not exactly the portrait of the movie star."