diff --git a/lib/google/api_client.rb b/lib/google/api_client.rb
index ce0a97a10..1f51324bf 100644
--- a/lib/google/api_client.rb
+++ b/lib/google/api_client.rb
@@ -265,13 +265,13 @@ module Google
         api_method = self.discovered_method(
           api_method.to_s, options[:service_version]
         )
-      elsif !api_method.kind_of?(::Google::APIClient::Service)
+      elsif !api_method.kind_of?(::Google::APIClient::Method)
         raise TypeError,
-          "Expected String, Symbol, or Google::APIClient::Service, " +
+          "Expected String, Symbol, or Google::APIClient::Method, " +
           "got #{api_method.class}."
       end
       unless api_method
-        raise ArgumentError, "API method does not exist."
+        raise ArgumentError, "API method could not be found."
       end
       request = api_method.generate_request(parameters, body, headers)
       if options[:signed]
diff --git a/lib/google/api_client/discovery.rb b/lib/google/api_client/discovery.rb
index 261a20caa..8976d2f9a 100644
--- a/lib/google/api_client/discovery.rb
+++ b/lib/google/api_client/discovery.rb
@@ -377,16 +377,74 @@ module Google
       #
       # @param [Hash, Array] parameters
       #   The parameters to send.
-      # @param [String] body The body for the HTTP request.
+      # @param [String, StringIO] body The body for the HTTP request.
       # @param [Hash, Array] headers The HTTP headers for the request.
       #
       # @return [Array] The generated HTTP request.
       def generate_request(parameters={}, body='', headers=[])
+        if body.respond_to?(:string)
+          body = body.string
+        elsif body.respond_to?(:to_str)
+          body = body.to_str
+        else
+          raise TypeError, "Expected String or StringIO, got #{body.class}."
+        end
+        if !headers.kind_of?(Array) && !headers.kind_of?(Hash)
+          raise TypeError, "Expected Hash or Array, got #{headers.class}."
+        end
         method = self.description['httpMethod'] || 'GET'
         uri = self.generate_uri(parameters)
+        headers = headers.to_a if headers.kind_of?(Hash)
         return [method, uri.to_str, headers, [body]]
       end
 
+      ##
+      # Returns a <code>Hash</code> of the parameter descriptions for
+      # this method.
+      #
+      # @return [Hash] The parameter descriptions.
+      def parameter_descriptions
+        @parameter_descriptions ||= Hash[self.description['parameters'] || {}]
+      end
+
+      ##
+      # Returns an <code>Array</code> of the parameters for this method.
+      #
+      # @return [Array] The parameters.
+      def parameters
+        @parameters ||= Hash[self.description['parameters'] || {}].keys
+      end
+
+      ##
+      # Returns an <code>Array</code> of the required parameters for this
+      # method.
+      #
+      # @return [Array] The required parameters.
+      #
+      # @example
+      #   # A list of all required parameters.
+      #   method.required_parameters
+      def required_parameters
+        @required_parameters ||= Hash[self.parameter_descriptions.select do |k, v|
+          v['required']
+        end].keys
+      end
+
+      ##
+      # Returns an <code>Array</code> of the optional parameters for this
+      # method.
+      #
+      # @return [Array] The optional parameters.
+      #
+      # @example
+      #   # A list of all optional parameters.
+      #   method.optional_parameters
+      def optional_parameters
+        @optional_parameters ||= Hash[self.parameter_descriptions.reject do |k, v|
+          v['required']
+        end].keys
+      end
+
       ##
       # Verifies that the parameters are valid for this method.  Raises an
       # exception if validation fails.
@@ -397,8 +455,7 @@ module Google
       # @return [NilClass] <code>nil</code> if validation passes.
       def validate_parameters(parameters={})
         parameters = self.normalize_parameters(parameters)
-        parameter_description = self.description['parameters'] || {}
-        required_variables = Hash[parameter_description.select do |k, v|
+        required_variables = Hash[self.parameter_descriptions.select do |k, v|
           v['required']
         end].keys
         missing_variables = required_variables - parameters.keys
@@ -407,8 +464,8 @@ module Google
             "Missing required parameters: #{missing_variables.join(', ')}."
         end
         parameters.each do |k, v|
-          if parameter_description[k]
-            pattern = parameter_description[k]['pattern']
+          if self.parameter_descriptions[k]
+            pattern = self.parameter_descriptions[k]['pattern']
             if pattern
               regexp = Regexp.new("^#{pattern}$")
               if v !~ regexp
diff --git a/spec/google/api_client/discovery_spec.rb b/spec/google/api_client/discovery_spec.rb
index f12fa12e3..2b6f77ad3 100644
--- a/spec/google/api_client/discovery_spec.rb
+++ b/spec/google/api_client/discovery_spec.rb
@@ -160,7 +160,7 @@ describe Google::APIClient, 'configured for the prediction API' do
     @client.latest_service('bogus').should == nil
   end
 
-  it 'should generate requests against the correct URIs' do
+  it 'should generate valid requests' do
     request = @client.generate_request(
       'prediction.training.insert',
       {'query' => '12345'},
@@ -169,13 +169,30 @@ describe Google::APIClient, 'configured for the prediction API' do
       {:signed => false}
     )
     method, uri, headers, body = request
+    method.should == 'POST'
+    uri.should ==
+      'https://www.googleapis.com/prediction/v1/training?query=12345'
+    Hash[headers].should == {}
+    body.should respond_to(:each)
+  end
+
+  it 'should generate requests against the correct URIs' do
+    request = @client.generate_request(
+      :'prediction.training.insert',
+      {'query' => '12345'},
+      '',
+      [],
+      {:signed => false}
+    )
+    method, uri, headers, body = request
     uri.should ==
       'https://www.googleapis.com/prediction/v1/training?query=12345'
   end
 
   it 'should generate requests against the correct URIs' do
+    prediction = @client.discovered_service('prediction', 'v1')
     request = @client.generate_request(
-      :'prediction.training.insert',
+      prediction.training.insert,
       {'query' => '12345'},
       '',
       [],
@@ -221,6 +238,12 @@ describe Google::APIClient, 'configured for the prediction API' do
       @client.generate_request(42)
     end).should raise_error(TypeError)
   end
+
+  it 'should raise an error for bogus methods' do
+    (lambda do
+      @client.generate_request(@client.discovered_service('prediction'))
+    end).should raise_error(TypeError)
+  end
 end
 
 describe Google::APIClient, 'configured for the buzz API' do