Film producers prepare and then supervise the making of a film before presenting the product to a financing entity or a film distributor. They might be employed by a film studio or be independent, yet either way they helm the creative people as well as the accounting personnel.
During the "discovery stage" the producer has to find and acknowledge promising material. Often a producer must then retrieve the film rights or an option. If the rights owner is worried about preserving the integrity, voice and vision of his/her work, the producer might have to comply with a variety of demands concerning the screenplay, the film director, the casts or other topics.  Thus it takes occasionally a lot of time and effort before the actual pre-production can begin. Late German producer Bernd Eichinger is said to have worked 15 years on convincing novelist Patrick Süskind just to agree to a film adaptation of his book Perfume. However, sometimes all effort is futile. J. D. Salinger, for example, refused all film producers as long as he lived.
Unless the film is supposed to be based on an original script, the producer has to find an appropriate screenwriter. If an existing script is considered flawed, they are able to order a new version or make the decision to hire a script doctor. The producer also has the final say on which film director gets hired. They also have the last word when it comes to casting questions. 
Among other things the producer has the last word on whether sounds or music have to be changed or scenes have to be looped. Even if the shooting has officially been finished, the producers can still demand that additional scenes be filmed. In case of a negative test screening producers might even demand and get an alternative film ending. This happened for example with First Blood.  Producers are also in charge of selling the film or arranging distribution rights.
Becoming a film producer
From the first draft script, through all stages of production, to the final dub, success or failure rests largely in the hands of the producer. Experience in this field does not come overnight. Rather it is born out of long years of creative and technical know-how, and above all a love for the job and all that goes with it, together with the ability to choose the right talent with which to surround himself.
There are different ways to become a film producer. Stanley Kramer started as editor and writer. Other producers started as actor or director.
Film schools offer degree courses that include film production knowledge. Some courses are especially designed for future film producers, focusing on key topics like pitching, script development, script assessment, shooting schedule design and budgeting. The students can also expect practical training in regards to post-production..
On the occasion of announcing his own film school "L’Ecole de la Cite" film producer Luc Besson admitted that at the beginning of his career he would have appreciated the chance to attend a film school.