The British "Regulars" were well trained and disciplined. The Minute Men spent the night before in a tavern. 

I took the day off today, but I up at the same early hour. I'm headed to Lexington Green for the annual reenactment of the dawn battle of April 19, 1775. If you're in a Patriot's Day state-of-mind, check out this essay from The Journal of the American Revolution entitled "Who Shot First? The Americans!" Here's a sample:

Consider too that many of these men spent the night drinking at Buckman Tavern, complaining about the British as they bolstered their bravery with alcohol. Can we be sure alcohol did not play a role in the first shots? (And, were the first shots not from behind Buckman tavern, but inside Buckman tavern, in the form of West Indies rum?)  Thus, the Green was surrounded by disgruntled and possibly drunken militiamen of all ages and experiences, all armed, even as a few more disciplined militiamen formed up on the Green itself.

I agree with this premise, but no one knows for sure. Like many events in history, an understanding of human nature and behavior can help to fill the gaps. 

UPDATE: Click HERE for video of this morning's reenactment. Look closely in the background and you'll see two shots fired from a second story window of Buckman Tavern an instant before the battle.

Buckman Tavern, Lexington, Massachusetts