ボタンを押すたびに一行ずつ表示

Sample.cs

using System.Collections.Generic;
using UnityEngine;

public class Sample : MonoBehaviour
{
    [SerializeField] List<string> messages;
    [SerializeField] UnityEngine.UI.Text messageBox;

    private int current = 0;

    private void Start()
    {
        OnClick();
    }

    public void OnClick()
    {
        messageBox.text = messages[current];
        current = Mathf.Min(current + 1, messages.Count - 1);
    }
}