His secret job
This wasn’t the first time Phoenix has made the extra mile in order to perfect a role (and probably not the last time, either). One example is the film Ladder 49, where he stepped into the role of Jack Morrison, a Baltimore firefighter, and in order to fill those shows, completed a firefighters course. The actor attended the Baltimore Fire Academy, and so he is a certified fire fighter, among his other secret talents.
While it isn’t uncommon for actors to take courses and step out of their comfort zone in preparation for roles, Phoenix was always just a little bit ahead. His Ladder 49 co-stars, including John Travolta and Robert Patrick attended the Baltimore Fire Academy as well, but Phoenix was the only one who took it seriously, never missing a class, being the first one to arrive and the last one to leave, completing six weeks in total of training and working out of an actual firehouse.