gyllenhaal lost 30 lbs for thriller nightcrawler then gained it all back - and put on 15 more pounds of pure muscle in six months .
went to the gym twice a day for six hours .
regime included 2,000 situps , three hours of boxing and three hours of cardio and weight training .
mimics great hollywood physique changes like matthew mcconaughey going from magic mike hunk to dallas buyers club aids patient .