String to Object

I have this function that receives a string which has the name of the movieClip I am attaching:

function contentsFadesin**(movieFadesin:String)**:Void {
attachMovie(movieFadesin, movieFadesin,1);
lastSelectedMovie._visible =false;
movieFadesin._x = 369;
movieFadesIn._y = 54;
movieFadesIn._visible = true;
movieFadesIn._alpha = 2;
movieFadesIn.onEnterFrame = function() {
if (movieFadesIn._alpha<=100) {
movieFadesIn._alpha += movieFadesIn._alpha*1;
} else {
delete this.onEnterFrame;
lastSelectedMovie = movieFadesIn;
}
};
}

The movieClip is attached just fine, but then the rest of the function should fade the movie in. The problem is: the variable “**movieFadesin” **is passed as a string (so I can use attachMovie), but the rest of the function only accepts and “Object” variable in order to work properly.

My questions is how can I change the string value of **movieFadesin **into a object?