Mild fear here, I can talk in groups of people just fine, but I get nervous before and during a presentation (something for which I have taken deliberate steps to get better at).

For me at least, the primary thing that helps is being comfortable with the subject matter. If I feel like I know what I'm talking about and I practiced what I am going to say it usually goes fine (it took some effort to get to this level, btw), but if I feel like I have to bluff my way through everything falls apart real fast. The number of people in the audience and how well I know them both have noticeable effect as well, but what the audience is doing has almost no influence at all.

The one exception to this is asking questions, if I have a good answer to a question my mind switches from presentation mode to conversation mode, which I am, for some reason, much more at ease with. (Note: This doesn't work on everyone, some people instead get way more nervous, so don't take this as an encouragement to start asking questions when the presenter seems nervous.)