You should know at least the size, age, and education and experience of your group before you prepare your talk. One way to do this is ask your host or simply ask the audience some questions before you start. Will you be talking to a group of five, 50, or 500? This may not impact what you say, but it may impact how you say it and what you bring with you. A hands-on demonstration planned for a group of five will not work so well with a group of 50. So knowing the audience is critical.
Subject interest and knowledge of the audience is also a key thing. Just because people are attending the talk does not mean they know anything about the subject, or that they have the same level of understanding of the subject you do. Speakers are typically experts on their subject but do not mistakenly assume that the audience has the same level of knowledge, education, and background information. This is not true at all times.
Do the people in your audience have a high school diploma, two-year technical school degree, Bachelor 's or Master 's degree, or a Ph.D.? You need to talk differently to each group. You may need to dress differently for each group as well.
Another important thing that you know about your audience is their experience and expectations. There is a difference between knowledge and experience. Someone might be very knowledgeable about something (through reading), but may never have actually experienced the topic first hand. You may or may not be talking about something with which the audience has actual experience. If they do, that experience may not be positive. You could potentially be walking into a minefield. Ask the right questions
References: Gilman, S. (2012). Keeping your audience engaged. RDH, , 40-43. Retrieved from http://ezproxy.bellevue.edu:80/login?url=http://search.proquest.com/docview/1027140604?accountid=28125 Gruhn, P. (2009). Speaker, know thy audience. INTECH, 56(8), 41-41. Retrieved from http://ezproxy.bellevue.edu:80/login?url=http://search.proquest.com/docview/208827362?accountid=28125