In Edmund and the sisters, Shakespeare has created fully rounded characters, more than one-dimensional villains. Notice that they all have real grievances against their fathers. Edmund has always been treated as an embarrassment and a lesser son than Edgar, due not to any inherent failings but to his illegitimacy, the fault of his father's adultery. Goneril is forced to put up with the riotous company that Lear keeps at his side. In the 1962 Peter Brook production  (and in the 1970 film) these "men of choice and rarest parts" (as Lear claims) overturn tables and wreck Goneril's home (in one rehearsal the actors became so violent that they brought a chandelier crashing to the floor, to the chagrin of the stage manager).

The curse that Lear places on Goneril reminds us of his rash rejection of Cordelia in scene one. To this point Goneril has not mistreated her father or threatened his life, but has simply insisted that he practice a little restraint, to which Lear explodes in terrible rage. "The question is not whether Goneril deserves these appalling [curses] but what they tell us about Lear. They show that, although he has already recognized his injustice toward Cordelia ... the disposition from which his first error sprang is still unchanged." (A. C. Bradley, Shakespearean Tragedy. Fawcett, 234)

The part of the Fool was usually cut from productions in the 18th and 19th centuries, as critics thought that this "comic" role was out of place in the tragedy. However, unlike most of Shakespeare's clowns, the Fool should not be interpreted as humorous. He acts as the voice of Lear's conscience, telling him truths that no one else dare speak to his face (as witnessed in Lear's rejection of Cordelia and Kent's blunt criticism in scene one). Almost every line of "nonsense" the Fool speaks has a definite point, reminding Lear of his own folly. He describes a topsy-turvy world where men carry asses on their backs and fathers must obey daughters (text). He echoes Lear's own words "Nothing will come of nothing," (I.i) turning their sharp point back on Lear himself (text): how can he expect the reverence he once knew now that he has given away his authority? 

In the Lawrence Olivier video, John Hurt plays the part with great feeling and sympathy, deeply concerned for Lear's welfare. 

Most likely, in the original production the part of the Fool was played by the same boy actor that doubled as Cordelia, as they are never on stage together in the script.