How do the differences between what characters think and what they say add to the humor of Lucky Jim?

One of the models on which comedy operates in Lucky Jim is incongruence, or the collision of two very different elements. We are introduced to this mode of humor in the first chapter of the novel when Dixon and Professor Welch walk across campus talking, and then ride to the Welches' in the Professor's car. The first incongruence revealed is the vast difference between Dixon's thoughts and Dixon's speech—while Dixon makes short, bland comments to the Professor, his actual thoughts are lengthy, perceptive, and mocking of both the Professor and his chosen conversation topic of amateur music recitals. Another, related incongruity that becomes apparent in Chapter 1 is the disparity between Dixon's position at the college and his feelings about scholarly work. When Dixon thinks of his recently written academic article, he thinks of its "niggling mindlessness." The very basic incongruity between the career that Dixon is trying to secure himself and his feelings about that type of career is indicative of a level of comedy that continues throughout Lucky Jim.

What pivotal distinction does Dixon learn to make at the Summer Ball? Is this new way of seeing the world consistent, or does Dixon sometimes lapse from it?

Dixon makes the statement about dividing the world between people he likes and people he doesn't like during his ride home with Christine from the Summer Ball. The Summer Ball marks the first climax of the novel—it is the moment when Dixon begins to learn things about himself and change things about himself. The statement itself, with its clear-cut and overly simplistic divisions, reflects Dixon's newfound decisiveness in his actions with Christine at the Ball. Before the Ball, and indeed, for select moments afterwards, Dixon is not at all decisive about dividing the world up into people he likes and doesn't. Margaret, for one does not fit cleanly into either of those categories for most of the novel. Nor, eventually, does Professor Welch, whom Dixon does not fully dislike by the end of the novel.

Is Dixon's lecture a failure or a success? In what ways does Dixon achieve something out of his first public speech?

Dixon's speech, and specifically his imitation of Professor Welch and the Principal, could be read as Dixon's performance of his true talent, and therefore a triumph rather than an embarrassment or insult. If Dixon had given the "Merrie England" lecture straight, as he had originally written it, with Professor Welch's subject preferences and speech mannerisms in mind, the lecture would have been an embarrassing failure for Dixon. Instead, Dixon takes the lecture and makes it his own in a way that only Dixon could by mimicking the linguistic nuances of others. It is Dixon's special talent to pick up on and analyze the tone, mannerisms, and pitch of others' speech, especially those he doesn't like. By performing this talent, Dixon finally asserts his individuality, an individuality that had been lost on his college colleagues.