Internal struct variables when casting to an interface

gistfile1.txt

// Variables accesible directly on the structure
// but after casted to a interface, still accesible
// for the implementing methods thought

package main

import "fmt"

type Animal interface{
  Speak() string
  Walk() string
}

type Cat struct{}

func (c *Cat)Speak() string {
  return "Miau"
}

func (c *Cat) Walk() string{
  return "Kittie walk"
}

type Cat1 struct{
  specialty string
}

func (c *Cat1) Speak() string{
  s := "Speaking " + c.specialty
  return s
}

func (c *Cat1) Walk() string{
  s := "Walking " + c.specialty
  return s
}


func main() {
  c := &Cat{}

  fmt.Println(c.Speak())
  fmt.Println(c.Walk())

  c1 := &Cat1{"especially for you"}

  fmt.Println("Specialty", c1.specialty)

  g := Animal(c1)
  fmt.Println("Cat casted to Animal", g.Speak())
  fmt.Println("Specialty", g.specialty) //g.specialty undefined (type Animal has no field or method specialty)
}