Rigidbody controller and slopes

I used a capsule collider with a rigidbody character, and it could climb steps up to half its height. My script is as follows:

 var horSpeed: float = 6.0;
 var jumpSpeed: float = 8.0;
 
 function Start(){
     // no physics rotations, please!
     rigidbody.freezeRotation = true;
 }
 
 function Update(){
     // get the direction it must walk in:
     var speed = Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical")).normalized;
     // convert from local to world space and multiply by horizontal speed:
     speed = horSpeed * transform.TransformDirection(speed);
     // keep rigidbody vertical velocity to preserve gravity action:
     speed.y = rigidbody.velocity.y;
     // set new rigidbody velocity:
     rigidbody.velocity = speed;
 }

This is a basic movement script, and emulates the CharacterController behaviour - maybe it works as a starting point in your case. This script only moves forth/back and sideways - you can add a MouseLook script to the character with Axes = Mouse X to rotate it with the mouse, and another MouseLook to the camera (if it's a first person game) with Axes = Mouse Y to look up and down.
But I made once a simple "Gravity Guy"-like script using a CharacterController: it works in flipped gravity as well because the CharacterController is always vertical, even if we rotate the transform - the only problem is the property isGrounded, which will always be false when upside down.
If you want to walk on any surface instead - floor, ceil, walls etc. - use this script.