Actors

Jamie Foxx

Jamie Foxx said in an interview that he took the role of Nathaniel as it mirrored something he went through in his childhood. As a child he had a fear of going crazy and of hospitals. When he was in college he took some drugs and had to go to hospital and was ill for almost a year; he had a friend who spoke to him every night and calmed him down. This is much like The Soloist as Downey Jr's character looks after Foxx's everyday.
When taking the role his fear came back strong and he was frightened that he could 'catch' schizophrenia and get ill. For this reason he employed a therapist whilst getting into the role as he wanted to become the character and delve deep into Nathaniel but with the help of a therapist to bring him back from the brink of going crazy. The therapist also helped with his fear by telling him what he felt was post traumatic stress from his hospital trip; not schizophrenia.
This role was a diversion from his usual roles as he usually plays parodies of other characters or people in real life; he also writes and produces more often than he acts.

Robert Downey Jr

Downey Jr was, for a change, a secondary character in this role. He said in an interview that he was simply another pair of eyes to show Nathaniel's character through and that Foxx's performance was exceptional. He was asked to play this role by the director but may have chosen to accept a secondary role as he was busy working on Iron Man 2 and Sherlock Holmes at the time of production. He also enjoyed working on this film as it is very contemporary and deals with issues that we still face today. For example it helped Robert to better understand the homeless and the role that they play in this world - he spent a lot of time with the homeless to help understand them and how to interact with them.