_controller.rb

def new
  # supply the default values to the new form. 
  # no params are getting posted to this so no need to look in params[]
  @comment = Comment.new(default_attributes)
end

def create
  # first step, make a new comment with the stuff posted to your action, but with the default attributes
  #
  # hash_a.merge(hash_b) gets every element of hash_b to override elements with the same key in hash_a,
  # but those only in hash_a or only in hash_b to stick around
  @comment = Comment.new(params[:form].merge(default_attributes))
  if @comment.save
    # magic
  else
    # less magic
  end
end

private
# always best to have it as a method. i'm not sure if a local variable would work there.
# this does the same but has better scope
def default_attributes
  {:name => cookies[:name], :email => cookies[:email]}
end