Okay, final draft.
Control
By control, I don't simply mean being able to move your character to the desired position. Control, in the most general sense when it comes to Smash, means that you have effectively precluded the possibilities of your opponent. Movement, spacing, shielding, punishment and all other aspects of competitive Smash are avenues for you to impose your will on your opponent, or in other words, control the match.
A friend of mine said this to me years ago: "Smash is not a game of damage, it's a game of positioning." It took me a while to understand what he meant, but it's made obvious by the term gimp. We all know what a gimp kill is, but it's made possible only through superior positioning. If you're Fox and your opponent is in prime position for a shine spike, then the damage is irrelevant; they are are in a place where the course of the match is now out of their hands, and they must rely on the Fox player to make a mistake. That's where you want to be: you want your opponent to hope that you make a mistake.
Remember though, positioning not only refers to physical placement on the stage, but also the mental position you've placed your opponent in. Mix-ups and utilizing different attacks, varying approaches, de-syncs and shield tricks should be used to fluster and confuse your opponent, so that they make that mistake that will land you a grab. When you place your opponent in a position where he doesn't know what you're going to do next, his options are severely limited. He can no longer predict and act, he can only react. That puts the initiative in your hands, and allows you to control the match.
As IC mains, it's imperative that we win the positioning battle, both physical and mental. As Wobbles pointed out, you will usually be playing from behind in most circumstances, as all of the top and high tiers can eat the IC's lunch if given the opportunity. We don't have any "home run" attacks like Fox's shine or Falcon's knee (sorry Binx, couldn't help it ), so if a match turns on a dime, it will not be in your favor. Limit your opponent's options. Put him in an unfavorable position. Exert your will over him. Control the match, and control your own destiny. It's the only way to win.