Perhaps your image of this person is that they are gregarious… that they always have something to say… always have the right thing to say… that they have never met a stranger… that they are outgoing and love people.
While this could be true of some evangelists, there is nowhere in scripture that describes an evangelist this way. They obviously have the ability to communicate the gospel well and seem to used of God to be there when people step from darkness to light, but their primary job according to Ephesians 4:11-12 is that they are supposed to encourage and equip the rest of the congregation to do better at evangelism.
Some evangelists are quiet and shy. Some are dynamic and extroverts. Some are dear old ladies and others are fiery young men. There is not normal personality for an evangelist but you will discover them by their actions. They are able to inspire and instruct others to do reach the lost. There is something contagious about their stories and their boldness. They quietly motivate and model evangelism. So stop looking for demeanor and look for their demonstration of effective outreach to lost people.