Being commissioned by a client to do a specific person will always be a challenge for an artist. The reason for this is that rarely is an artist allowed total freedom of expression to depict the subject totally as he or she sees it.
One starts off by researching the character (especially if it's historic) by means of on-line research, books, audio-visual films and by interviewing next-of-kin or acquintances. Obviously if the subject is still alive this whole process is a lot easier as he or she is the artist's ultimate reference.
Then one has to consider the client's brief - what aspect of this person you need to depict etc. It is always a learning curve as the treatment of any two characters will never be the same.