class SomeController < ApplicationController
  def some_methode
    some_var = some_value
    render text: some_var
  end
end

assign(:some_var, some_other_value )
controller.some_methode.stub(some_var: some_other_value )
some_methode.stub(some_var: some_other_value )
controller.some_methode.stub!(some_var: some_other_value )
controller.some_methode.stub(:some_var).and_return(some_other_value )

it "assigns some_value to some_var" do
  get :some_method
  assigns(:some_var).should == some_value
end

def some_method
  some_var = get_value
  render text: some_var
end

private

def get_value
  some_value
end

it "does something when some_var is assigned value_to_test" do
  controller.stub(:get_value).and_return(value_to_test)
  get :some_method
  ... should ...
end

it "does something when passed value_to_test" do
  get :some_method, :some_var => value_to_test
  ... should ...
end