What game, what version? Be specific.
You can change key bindings under Options / Controls in T1, T2, and T3 Deadly shadows. I don't know about T4, but I assume so. R-mouse is one of the keys you can bind to an action such as walk or run forward. Before doing that, look down the list to see what R-mouse is currently bound to. You may have to change a number of things as you work your way backward.
I tried it in T2, and it worked.
"What we've got here is a failure to communicate", the warden (Strother Martin), Cool Hand Luke