Answer:
Well, it depends. Are you directing/producing a movie and you need staff (ex. costume designers, production assistants, film editors/writers, screenwriters)? In that case, you can either write, call, or even arrange a meeting (it can also be over coffee or dinner) to meet with the person in charge of the company you want to work with.
Hope this helps! Although it would have been better if you could have been more specific in your question.
P.S. Some Advice: Be as polite as possible. Some can be really impatient and arrogant. Good Luck!