I wanted to be Will, not in The Dark Is Rising, but in Silver on the Tree, so that I could make them not do the last five pages, so that I, girl-Will, could rise up and start smiting the Old Ones and the Dark alike. The last five pages of that book are so not canonical. The Light are right bastards and, what's worse, typical grownups.
I wanted to be Faith Meredith so I could convince Walter to train for a medic or something and not go proving his stupid bravery for me. I wanted to be Ilse Burnley so I could send Teddy Kent packing away from both me and my best friend Emily of New Moon so that we and Perry could do cool stuff without his clueless spineless (brainless! hopeless!) mommy-obsessed pretentious-artist self, and then maybe Emily would get over her pretentious artist stuff, too. Also maybe Teddy could be unemployed in Greenland. That would have been all right.
I wanted to be Vicky Austin so I could knock sense into Adam Eddington halfway through the book (A Ring of Endless Light) instead of at the very end and give Zachary Gray the boot on day one of The Moon By Night. And apply said boot as many times as necessary until he got the point. I wanted to be Polly O'Keefe so I could knock sense into Renny at all (in A House Like a Lotus).
I wanted to be Aslan so I could stop kicking people out of Narnia. (Also, girl-Aslan. Like many of the people answering this meme, I wanted to replace the most interesting or effective character to replace; unlike many of the people answering this meme, my self-concept has always been firmly gendered, so Aslan would just have to be a girl if I was Aslan.)
I wanted to be Princess Leia so I could thief somebody's lightsaber and take care of business. I wanted to be Princess Buttercup so I could poison Prince Humperdink's tea, take the throne myself, and, after a period of decorous mourning, marry Westley.
Basically, I was fairly convinced that fictional characters in movies and children's books did not kick enough ass, or did not kick ass in the correct directions. I have said before and will say again: when I was a kid, I had a very firm awareness that writers were just people like me, only sadly less competent.
When the characters I read kicked ass, I wanted to invite them out to fix things here. Sir Percy Blakeney, for example, would have been allowed to smuggle me out of Blumfield Elementary in a cart of cabbages whenever he pleased. That would have been quite all right with me.