如何从Go中的struct的接口实例获取属性

I want to get the v.val, but the go compiler throw me an Error:

v.val undefined (type testInterface has no field or method val)

but in the v.testMe method,It work.

package main

import (
    "fmt"
)

type testInterface interface {
    testMe()
}

type oriValue struct {
    val int
}

func (o oriValue) testMe() {
    fmt.Println(o.val, "I'm test interface")
}

func main() {
    var v testInterface = &oriValue{
        val: 1,
    }
    //It work!
    //print 1 "I'm test interface"
    v.testMe()
    //error:v.val undefined (type testInterface has no field or method val)
    fmt.Println(v.val)
}