After not being able to find a way to display messages in a text box (for things such as characters talking and messages displaying on screen), I decided to write my own script to do this. The code I have at the moment is below:

var MessageBoxSkin : GUISkin;
var MessageBoxText : String;
var showText : boolean = false;


function DisplayMessage(message : String) {
        print(message);
        showText = true;
        MessageBoxText= message; 
}


function OnGUI()
{
    if (showText)
    {
    GUI.BeginGroup (new Rect (Screen.width / 2 - 640, Screen.height / 2 - 360, 1280, 720));

    GUI.skin = MessageBoxSkin;

    GUI.Box (new Rect (40, 600, 1200, 100),
    MessageBoxText);

    GUI.EndGroup ();
    }
    else
    {

    }
}

function Update(){

if (Input.GetKeyDown(KeyCode.F) && showText)
    showText = false;
}

function Active(){
return showText;
}

I have the MessageSystem attached to an empty gameobject in the scene, and I get other scripts to call the MessagesSystem via Get Component.

I can get the MessageSystem to receive the message (in the DisplayMessage function) but Update and OnGUI never seem to get called, and thus the GUI Box never appears.

Any help?