Writing in Textbox freezes form

Question

I have a form that has a Textbox and would like to write in it but give an effect of Typewriter so that the text is not added fully "dry".

I created a method that takes the text I’d like to add and does the effect as expected. However, the form is frozen and only after the writing of all the text is possible to use the form.

    private void escreverMensagem(string msg)
    {
        char[] msg_array = msg.ToArray<char>();

        for (int i = 0; i < msg_array.Length; i++)
        {
            txtMensagens.AppendText(msg_array[i].ToString());
            Thread.Sleep(30);
        }
    }

When I need to add something I will just call it with the desired string.

I believe I should start a new Thread to make the method typeMensage(string msg), but I have no idea what it would look like.