People are always walking up and interrupting me when I'm talking to other people. I have yet to figure out what causes it - whether it's because I'm too quiet when I speak, or I'm not making enough obvious eye contact. Whatever the cause, I do find it very frustrating. I don't talk much, unless you get me onto a topic I really care about, so if I am talking, it's kind of a big deal, at least to me. To have someone blithely wander up and interrupt without apologising or without even realising that I'm in a conversation is very insulting.
Mokalus of Borg
PS - If I were being aggressive about it, I'd make physical contact while speaking.
PPS - That should leave no doubt about whether I'm speaking to someone, right?