Matthew McConaughey's mind was "hyper acute" after he shed almost 50lbs (22kg).
The 44-year-old actor shed 47lbs to portray AIDS victim Ron Woodruff in new movie 'The Dallas Buyers Club', and while he admits he struggled with physical tasks due to the weight loss, he believes it made him mentally more alert.
He said: "I had one scene where I had to run from gamblers, damn that was hard. Before, I could have done that 500 times.
"The first time I took off running I made it from here to that door. My legs locked up and - bam! - I crumpled and went down. I went, 'Time out!'
"That was kind of painful - I didn't know my legs had gone into that much hibernation. I was pretty damn weak.
"But the energy I lost from the neck down got transported to the neck up. My mind was kind of hyper acute and aware and sharp."
'The Lincoln Lawyer' star has seen his career go from strength to strength in recent years and says this is down to a desire to challenge himself rather than take on easy roles.
He told the Detroit News newspaper: "It started by me saying no to certain things. I said no to some action adventures, I said no to some action comedies. I said no to romantic comedies. I'd been doing them, they're fun, they come with a damn good paycheque, but I felt like I could do them tomorrow."