Before writing a single slide, every effective speaker defines one core purpose -- to inform, to persuade, or to inspire -- because a talk trying to do all three at once usually accomplishes none of them clearly. Understanding your audience's existing knowledge level prevents two common failures: over-explaining basics to experts (which feels condescending) or using jargon with newcomers (which loses them in the first two minutes). A useful exercise is writing a single 'core message' sentence -- what you want the audience to remember if they forget everything else -- before building any content, since this sentence becomes the filter for deciding what to include and, more importantly, what to cut. Professional speakers also research the practical context in advance: room size, time limit, whether the audience will have phones or laptops open, and what talk comes before or after theirs, because these details change tone and pacing choices.