I don't see introversion as a barrier to initiating conversation (even if one is less inclined overall) and it might be characteristic for most ESIs, I dunno, but personally I need something specific to talk about or some excuse to be pushed together cuz it's hard to come up with a subject out of nowhere or randomly ad lib stuff. So my "aggression" begins with creating some pretext like a gathering with friends where we'll just end up talking, rather than walking up to strangers and saying hello. (I'm all about opposing the idea that we sit on our hands waiting for our extroverts to drag us out of a hole, though. Ive almost always done the asking out and stuff, even if talking to a stranger needs planning)