Key to capturing the character of Kitty is to identify how she begins to differ from her sister, Lydia, who is definitely the leader of Kitty, after Lydia returns married. Let us remember that at the end of the novel, Kitty is "civilised" if that is the right word by the good influence of her two eldest sisters. I personally think that the audience in a stage version of this excellent novel should be able to see this process beginning when Kitty is separated from Lydia due to her marriage. Of course, there will be tears and anger from Kitty at the way that Lydia has married and gained a higher social status because of that, but actually being separated from Lydia allows her to develop her own character and to escape the bad influence of Lydia. I would suggest that the person playing Kitty thinks about modelling herself more on Jane after this stage, as this is the beginning of her process of maturing as a character.