The first impression should stay open long enough for a second one to teach it something.

First impressions form quickly and then start defending themselves. Once you have decided somebody is sharp, cold, warm or careless, later evidence gets bent to fit, and a second meeting is read through the first rather than alongside it. Holding an impression loosely is not naivety but room left for information you did not have yet. People are consistently more various than an opening minute suggests.

Pick one person you decided about early and enter the next conversation looking for what does not fit. Let a second impression correct the first rather than confirm it.