Help with Invisibility script.

You need to check the keyup and keydown events every frame in order to perform any action. The Update function is called by Unity automatically each frame, so just do t$$anonymous$$s in Update, like t$$anonymous$$s:

function Update() { if (Input.GetKeyDown ("q")) { //GetComponent(MeshRenderer).enabled = false; GetComponent(MeshRenderer).material.color.a = 0.3; }

 if (Input.GetKeyUp ("q"))
 {
     //GetComponent(MeshRenderer).enabled = true;
     GetComponent(MeshRenderer).material.color.a = 1;
 }

You should try some basic scripting tutorials and read into the Unity docs.

edit
I changed the example. Now you need a transparent shader on your character. The colors alpha value (color.a) specifies the opacity (1.0 == 100% opaque; 0.0 == 100% transparent).