character switching on collision

First, you should add a collider to the ball and each character, and change the `Is Trigger` option for each character collider to on, using the inspector.

Then, create a script, attached to each GameObject, which uses `OnTriggerEnter()` to detect collisions.
Inside each character's OnTriggerEnter() function, you should insert code which would turn off the main camera and on that character's individual camera.

A global variable could be used to determine which GameObject (ball or character) should respond to user interaction (controls). It could be something as simple as a variable which stores a GameObject.

Controls.js

 static var currentObject : GameObject = GameObject.Find("Ball");
 // stores the current GameObject, which is set to the ball GameObject in the beginning.

Character.js

 static var ball : GameObject = GameObject.Find("Ball"); // store the ball GameObject for use later...
 
 function OnUpdate() {
     if (Controls.currentObject == gameObject) { // if this is the current object
         // respond to controls
     }
 }
 function OnTriggerEnter(other : Collider) {
     if (other.gameObject == ball) { // check for collision with ball
         Controls.currentObject = gameObject; // transfer focus to me!
         // set camera to this GameObject's camera, etc.
     }
 }

I will leave the implementation details up to you, unless you need help, of course, in which case I will gladly help you.

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Edit

First, create a new JavaScript script and name it something like "Character.js". Inside of it, after the automagically created OnUpdate() function, type the following:

 function OnTriggerEnter(other : Collider) {
     if (other.gameObject == ball) { // check for collision with ball
         Controls.currentObject = gameObject; // transfer focus to me!
        // set camera  to this GameObject's camera, etc.
     }
 }

In place of // set camera to this GameObject's camera, etc., turn the main camera off (find the camera component using GetComponent(), set its .enabled = off) and a camera attached to the current character on, using similar methods.

For the creation of a global variable, create a new script (JS again) and name it something like "Controls.js", and copy the code from above into it. Using this, you can reference its variable, which by placing a static keyword in front of it, is global, by using something like Controls.currentObject. As in the OnUpdate() function I included above, you can check whether this object will respond to movement, and respond accordingly, by placing an `Input.GetButton()` inside. Put that function inside "Character.js".