The Laughing Cavalier
Hals was a genius. Painting mostly alla prima (painting directly on the canvas with no underpainting), he was able to capture the subtle details and quirks of a personality, throwing the portrait world for a loop. His commissioned portraits are more finished, and done in more than one sitting, but the freshness is still there.
Using a limited palette, he sketched in (very quickly) the shadows and masses of the face and then began immediately to apply color, instead of the traditional technique of finishing a complete monochrome version of the painting before applying color.
Here, one of his more famous paintings, The Merry Drinker. Most people would have a hard time capturing this feeling of life with a simple pencil sketch, but Hals did it again and again in full oil paintings. Again, I think he was a genius. Because of his method of painting, it's very hard to come by sketches by Hals--most of his work was done directly on panel or canvas.
And here's his wonderful St. Matthew. The feeling of humanity and paternal warmth of the saint comes through in a great way. Many other painters who I like, for example, El Greco, would have probably painted this very differently, showing the Saint as a sad but stern man. But with Hals, you feel the approachability of the Saint, as he reads from his Gospel to a small child.
Some of Hals' portraits show people who don't look likeable--they look arrogant or have a hint of cruelty to their eyes or mouth. But they look human, not like idealized versions of themselves. And they all look like people you might encounter today, walking down the street or shopping at the grocery store. That's Hals genius--making his subjects seem alive for generations yet to come.
You can see a lot more of Hals at the Wikimedia Commons here.