The following is an excerpt from the GM's section of my work in progress, big-scale, spear-em-up RPG, Thousand Spears. Although it's part of this game I can see it applying to most systems.
Playing Outside the Rules
The most important thing to remember about Thousand Spears is that it is an RPG and not a wargame. You could play it as a wargame, but if you do you’ll be missing out on what makes RPGs so special. In an RPG the GM and players are able to step out of the rules of the game to do, quite literally, anything.
As the GM it's up to you to maintain a balance of playing within the rules and playing outside the rules. This doesn't mean breaking the rules, but is everything the players do that isn't directly linked to a rule. For example, using an individual's Ability to raise some attack dice is playing firmly within the rules, but describing how the archers are pouring arrows onto the enemy is playing outside the rules.
In the majority of games more time will be spent outside the rules than inside them. There are no rules for your Inheritor giving a rousing speech to his men, deciding how to persuade the dock-master to loan him a ship or deciding whether the mysterious Ysorn can be trusted to keep a promise. The temptation may be to play out these situations as Trials or Challenges, but it's important to remember you're not bound to using a rule for everything.
The rules are there for a reason, and there is great fun to be had using them to act out huge battles, dramatic one-on-one duels and life or death journeys across hostile landscapes. Just stop each time you're about to step back into the rules to consider whether you really need to.