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 sculptor allowed total freedom of expression to depict the subject totally as he or she sees it.
I start 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 if possible. 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 subjects will never be the same.
It is important for the sculptor to have enough time to obtain a decent likeness. One cannot "trace" a sculpture as it is effectively 360 degree portraiture that needs to be accurate from all angles. This is really where "luck" comes in.... when one is fortunate enough to find the likeness early on... fortunate enough to have decent source material.... and fortunate enough to have a Client who gives one enough time to do the work properly... Clients who respect the sculptor's creative process enough to provide sufficient time to achieve the goal. Not every likeness happens quickly and one often needs the luxury of a few days away from the work so that one can get a fresh perspective.