How do I add a force to a rigidBody to move it backwards?

Trying to make my character walk backwards by getting a velocity vector in the opposite direction:

Direction = _character.transform.forward * -1;

Then applying it to my character's rigidbody:

_rigidbody.AddForce(Direction, ForceMode.Impulse);

However, my character continues to walk forward.

What am I doing wrong?

 

Do you get an error with the assignment of Direction?
You will only be applying 1 unit of force to a Rigidbody which may be too little if it has plenty of mass;