In writing a public speech, it's important to keep your audience in mind. Part of the preparation for speech writing is in the analysis of potential audience members. This is where the popular "know your audience" mantra comes in. After a thorough analysis, a speech writer would prepare statements, verbal and non-verbal cues, tone, message, and mannerisms based off that analysis. Even with adequate preparation, it's possible a speech will fail in that it won't evoke its intended response.
So, why might that be the case? Let's explore an example to understand this phenomenon.
Suppose a person, Dan, is tasked to give a speech to senior students at a high school in a suburban area (not local). His job is to talk about the importance of education and the value of a college degree in 2016. In his pre-speech analysis, he learns 80% of the students are from Spanish-speaking households, 65% of students would be first-generation college students, and the most unifying groups in the high school are the basketball team and drill team. To prepare his speech, he recognizes the challenges the students may face in their home study habits (maybe their parents can't help with English papers or application essays), the obstacles they may face in applying for college (maybe their family doesn't really understand why a degree is important—after all, they don't have one and it's very expensive to get one), and the motivations that may already exist (they may be working toward a sports scholarship).
So, he writes his speech with all those important things in mind. The day of his talk, he walks in and takes the stage to give his presentation. Even though Dan has a great speech planned, something goes wrong. He starts with his first few slides and the audience is mumbling quietly. Some people are walking out of the room. Others are rolling their eyes. Why might the audience be so restless?
As it turns out, Dan is an older white gentleman dressed in a nice suit and, although he seems nice and professional, the students don't relate with him personally. The majority of the students are young, Hispanic, and from low-income neighborhoods - Dan has none of those characteristics. In fact, no amount of planning could have changed the reception of this speech because Dan was probably not the correct person to give the speech in the first place.