What decisions must the director make, and how should the final result be judged? Is there any danger in closing off the complexities and riches of the plays? Is it necessary to do this in order to create a performance?
The whole purpose of a play is performance for an audience. A play's intended is audience is live, so it is unnecessary for the play's director to become wrapped up in the minute nuances, but instead focus his/her attention on the major points that he/she wants the audience to get; because there is no "pause" or "rewind" button, the audience must be able to understand -- which includes hearing and seeing - the scene immediately. This is one of the reasons why Shakespeare's plays are so much about language and rhythm - it is much simpler to follow along when listening to a beat. (Think about how quickly you memorize lyrics to a song versus prose of the same length.)
There is a whole lot of action in any Shakespeare play which never happens in front of the audience. The Globe Theatre stage was small, and any major action--and certainly any battles or scenes requiring a large group of people--would not have been feasible on that stage. That's why so many times we get a messenger or some other speaker who tells us of those events.
To that extent, then, one difficulty a movie director would have to overcome is creating entire scenes with little information to work with. In the case of Macbeth, that would include the opening battle scenes, Duncan's murder, Macduff's family's murders, and the final scenes with the apparent thousands of soldiers ready to lay seige to the castle.
A director for the stage would have the most trouble with the same thing, but in an opposite way--today's audience wants to see those things, and to be true to Shakespeare they would not appear on stage.
The main problem for the screenwriter and film director is knowing how to make an original film adaptation and not simply a filmed version of the play.
Shakespeare's plays are meant to be heard and not necessarily seen in close-up. The beauty comes from the language, and they are very wordy. A director must know what to cut and what to keep. A staged play is two hours, and so is a filmed adaptation, but most films only use half the dialogue from the original text, so a director must substitute image for word without losing the beauty of the language.
A stage director can more easily adapt a play to the stage, its intended venue. Most directors want to put their own stamp on a production: to make Hamlet Branagh's Hamlet. So the stage director must achieve a balance between the original and the new and not try to wrestle too much with ownership. Cuts will need to be made and tech cues will need to be added, so the play often becomes more of a spectacle than a play.
Lead actors drive Shakespearean tragedies. Often, a stage director will be able to direct a play only if a high-profile actor has agreed to play the role. A director begins with an Othello or Hamlet in mind while pitching his ideas to producers. This actor is as important, or more important, than the director, so his role must be a good fit.
Arthur Miller said that "movies are meant to be heard, but plays are meant to be overheard." In other words, movies blaring, but plays are subtle; movies are about image; plays about words and ideas.
Imagery, illusion, and music are very significant role-player in almost all the Shakespearean plays. And, to giving the images and illusory figures proper dramatic on-stage effect and visualize them maintaining a minimum balance with the real text are really difficult. The music should be very appropriate and effective. The director has to remain a bit more concerned than always about the acceptability of the play and also should make it sure whether the play has passed the test of "suspension of disbelief" in the case of Shakespeare.
But most important thing is that, in modern days, Shakespearean dialogues are transformed into colloquial at an optimum level. And, how one would deal with the dialogues, that would be the most important criterion of the success of the movie or the stage show.