Upon communicating with your audience, your end purpose should not be what you want to do -- instead it should be what you want your audience to do as a result of your communication. You want your audience to act and react in some way. Communicating involves the audience. Effective communication is achieving this purpose and to communicate effectively you must adapt to your audience. Meaning, you must know your audience.
Knowing your audience and your purpose will help determine your strategy. So you will need to clarify these two items as best you can (possibly by asking others). For example, a public thesis defense includes your jury, your friends, your colleagues, and most likely your family. A mixed crowd means you will have to cater to each crowd. Although it would depend, again, on institutional culture. Some institutions require you to primarily address the jury, no matter who else is in the room, as it is your only chance to convince them of your worth. Conversely, other institutions see the defense as a chance to reach the wider public with the visibility of your work.
Obviously, audiences come in all shapes and sizes. They can be reasonably well know to you, as when you adress a letter to a specific person, or less well defined, as when you publish an article in a magazine. Whenever possible, however, distinguish between specialists and nonspecialists, and between primary readers and secondary readers.
How much your audience knows about the topic will vary. Specialists will want more detail and generalists will want less. Specialists will apply your detailed information delivered to their work. Non-specialists need basic information, simpler language, terms and definitions, and an expansive introduction -- requiring more interpretation, typically with conclusions drawn.
Indeed, specialism is relative. Even a scientific paper published in a journal, which you can see as a specialized publication, will likely be read by newcomers to the field who are less specialized. Even referees on the programme committee of a conference cannot have an equal degree of expertise in all the proposals they must evaluate. All of this to say that one should not assume that a specialised audience knows everything you know. Effective scientific communication whether in a presentation or in writing strives to be all inclusive, in that it tries not to exclude anyone. A well-written scientific paper makes sense, at least in its broad lines, to anyone with a scientific background.
In regards to your audience, context matters. In writing to a single person or to a small, well-defined group of people, you would be forgiven for getting straight to the point and jumping directly to the heart of the matter. The context would be known to your primary audience but could still be at a loss when reading it. And there is the chance that what you write will be read by a secondary audience in the future. The more effective way of communicating will be to be mindful of all other audiences that could read and consume what you present.