In his popular biographical novel about Michelangelo, The Agony and the Ecstasy, Irving Stone (1961) argues that the sculpture depicts David "before he entered the battle, when he decided that the Israelites must be freed from their vassalage to the Philistines.... Was not the decision more important than the act itself, since character was more critical than action? For him, then, it was David's decision that made him a giant, not his killing of Goliath." (p. 390)Indeed. I thought David serene. Check him face on: he's a scowling Sylvester Stallone-clone.
From the conventional side view of the statue that Stone undoubtedly had in mind, David does appear to be lost in thought as he gazes off into the distance. From the long-concealed frontal view depicted above, however, it is clear that the only decision David is making is when to release his stone and where to place it. Contra Stone, he is already committed irretrievably to the battle.
There's also a digital toy available on that wp for turning the David image around yourself, should you be minded to.