This might necessitate the start of a new thread, but here goes (and sorry if I am being deathly boring):
There were writers such as Preston Sturges, Billy Wilder, John Huston and George Seaton (often forgotten among the important writer/directors of Golden Age Hollywood, but one who at the time was regarded as part of the group) who wrote scripts and then ascended to the director’s chair. Sturges was the first, and he and Wilder both strove to be directors to protect their words (Mitchell Leisen was a bete noire for both of them, though he was a better director than either in terms of mise en scene).
Mankiewicz was also a writer who sought to direct his scripts: "I felt the urge to direct because I couldn’t stomach what was being done with what I wrote.” But there was an important distinction between Mankiewicz and Sturges/Wilder who most wanted to protect their words from what they felt were the capricious changes of Leisen and other directors.
Mankiewicz believed that a) the properly written screenplay had already been directed; and b) the first stage of a director’s job was to write the script. He maintained that writing and directing were two halves of the same job: filmmaker. He thought that a person who directs a screenplay she has not written actually re-directs the screenplay (again with the caveat that the screenplay was properly written in the first place).
So while Sturges and Wilder movies are often well-photographed by good/great cinematographers, Mankiewicz’s films are different–there is a connection between word and image that is rarely found in other movies of his time (he acknowledged Sacha Guitry as a forerunner, especially THE STORY OF A CHEAT). Mankiewicz would become a major influence on Godard, Rohmer and Fassbinder with regard to the dynamic relationship between word and image.
Three other directors–Preminger, Hawks, and Hitchcock–were closely involved with the writing of their movies’ screenplays. Hawks, in fact, often re-wrote on the set–he was committed to the dialogue suiting the characterizations of his actors which he understood to evolve as the film was being made (he was also known to shift a film’s emphasis during shooting when he found himself out of sympathy with one of his actors, e.g., HATARI and RIO LOBO). With Hawks films also, I think a keen dynamic exists between word and image.
The Hawks/Mankiewicz approach is the minority one in the present day. More and more film came to be understood as primarily a visual medium–dialogue serving as a handmaiden to the image. Directors may still work closely with their screenwriter(s), but it often it seems that they want a script that serves as a launching pad for visual spectacle–dialogue is to remain out of the way/functional and never interact with the image the way sound design/music does. But then again, the visual also seems to dominate in theatre today more than it has ever before (at least in my memory).
For me, I like films where the image is involved in a complex dynamic with dialogue and not just with the sound design (one of ALL ABOUT EVE’s never-bested record of 14 Academy Award nominations was for Best Sound Recording–which it won). Mankiewicz’s words and images are equal in terms of quality and precision; Hawks’ dialogue perfectly meshes with his plan americain mise en scene. Painterliness is more valued nowadays–Mankiewicz was right when he said in 1986 that “I’ve been in on the beginning, the rise, peak, collapse and end of the talking picture.” Movies still employ dialogue, but they rarely talk anymore.