Director Jean-marc Vallee almost laughed at the idea of casting Matthew Mcconaughey as an Aids victim in his new film Dallas Buyers Club because he couldn't imagine the movie hunk making the drastic transformation needed for the role.

The French-Canadian filmmaker's production team suggested the Magic Mike star for the gritty movie's lead, and Vallee admits he nearly rejected the actor simply based on his reputation as one of the sexiest men in Hollywood - but he's glad he didn't.

MCConaughey shed more than 40 pounds (18.1 kilograms) to portray real-life American cowboy Ron Woodruff, who smuggled life-saving drugs across the Mexican border into the U.S. after being diagnosed with Aids during the 1980s, and his dedication and focus on the job blew Vallee away.

He tells Details magazine, "I said, 'Really? Matthew for this? Beautiful, muscled Matthew for this?' But I've got to tell you, I've been witnessing the most spectacular, amazing, touching acting performance of my humble career so far. He had to create a new way of walking and being and not having this confidence of being handsome and seductive. I think people will see something different here, really a new Matthew MCConaughey. I think he wanted something new in his life, and you can see that in the choices he's made in the last two or three years."

MCConaughey's co-star Jennifer Garner, who previously worked with him in 2009's Ghosts of Girlfriends Past, was also impressed by the growth in his acting and insists he was a whole new man on the set of the gritty drama.

She says, "He is who he is, no matter what the project - friendly, funny and focused. But he was on another plane altogether making this film. I'm not sure if Matthew has changed from fatherhood, marriage, or just regular growing up - but he was more vulnerable and also more scary this time around."