attacker.rb

module Attacker
  def attack(target)
  end
end

character_attacks_another_character.rb

class CharacterAttacksAnotherCharacter
  attr_reader :attacker, :target

  def initialize(attacker, target)
    @attacker = attacker.extend(Attacker)
    @target   = target.extend(Target)
  end

  def execute
    attacker.attack(target)
    self
  end
end

character_attacks_another_character_spec.rb

require 'spec_helper'

describe CharacterAttacksAnotherCharacter do
  before do
    @luke   = stub
    @vader  = stub
    @attack = CharacterAttacksAnotherCharacter.new(@luke, @vader)
  end

  describe "constructor" do
    it "should assign the attacker and the target" do
      @attack.attacker.should be(@luke)
      @attack.target.should be(@vader)
    end

    it "should give roles to actors" do
      @attack.attacker.should be_an(Attacker)
      @attack.target.should be_a(Target)
    end
  end

  describe "#execute" do
    # This example tests the implementation. How to test the behaviour instead?
    it "should make the attacker to attack the target" do
      @luke.should_receive(:attack).with(@vader)
      @attack.execute
    end

    it "should expose the actors" do
      @executed_attack = @attack.execute
      @executed_attack.attacker.should be(@luke)
      @executed_attack.target.should be(@vader)
    end
  end
end

target.rb

module Target
end