For the purpose of my explanation, I will use this sample essay question taken from the OCR F663 June 2013 paper:
"Gloucester is no less than a tragic figure than his king" By considering the role and dramatic presentation of Gloucester in King Lear, evaluate this view.
After reading the essay title, the first thing that you should do is to highlight the keywords of the question. In identifying the keywords, ask yourself: what is the question really asking me about? Here, the examiner is asking you to compare and contrast the role and presentation of Gloucester with Lear - whether Gloucester is no less of a tragic figure. But this is only half of the question. More fundamentally, the examiner is asking what the term "tragic figure" entails? And how Gloucester and Lear fit into the definition of "tragic figure". Hence, I would suggest that the keywords of this essay title should be "no less" and "tragic figure".
Upon identifying the keywords, you can now consider the content of your essay - how are you going to tackle the question? What arguments are you going to put forward?
In structuring your essay, always start from the obvious, then extend your argument to the more subtle and discrete points. In the context of the sample question, the most obvious point would be the definition of "tragic figure". The term sets the parameters for the comparison between Gloucester and Lear, it sets the parameters of your essay. Discuss characteristics and possible interpretations of what a tragic figure is, then discuss individually how Gloucester and Lear fit into the description that you put forward. Then compare and contrast the two characters: are they both equally as tragic, or is one more tragic than the other?
Be careful when analysing the role and presentation of the characters, whilst it is important to offer a close reading by focusing on quotes/specific acts, do not loose sight of the play as a whole - relate the characters with the themes or structure of the play. For example, you can mention how Gloucester and Lear fit into the parallel plot structure of the play. Lear belongs to the main plot and Gloucester to the sub-plot. One of the role of the sub-plot is to mirror the main plot, highlighting key themes and development in the main plot. Extending on this logic, if Gloucester's tragedy overshadows Lear's, the aim of the parallel plot would be undermined. It would only make sense if Gloucester is less tragic than Lear as a character to achieve that effect most effectively.
Given that King Lear is a play, do not forget to take account into stage directions. In light of the sample question, an example would be the difference in dramatic effect between Lear's death and Gloucester's death. Lear dies onstage whilst Gloucester dies offstage; in this sense, Lear's death is more dramatic, highlighting its tragic element and emphasising Lear as the more tragic character.
Finally, once you have your arguments sorted out, to achieve top marks, you can then consider relevant critical opinions, literary interpretations and the historical/social context of the play.