Disclaimer: I'm a mere beginner at Unity and game development generally. I wrote this for myself as I am learning and figured others might find it useful as well. Exercise caution when running this code!

Using Unity version 2020.3.2f1 and based on this tutorial – with some changes – I want to show you how you can make your character do the following:

• Move forward/backward
• Move sideways and diagonally
• Sprint while holding left shift
• Rotate by moving the mouse

My code does not implement:

• Jumping
• Looking up or down
• Going up or down slopes

Here's the resulting code without comments (I will walk you through it step by step further down):

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class PlayerController : MonoBehaviour
{
  public CharacterController controller;

  void Start()
  {
    Cursor.lockState = CursorLockMode.Locked;
  }

  void Update()
  {
    Move();
    Rotate();
  }

  private void Move() {
    float moveZ = Input.GetAxis("Vertical");
    float moveX = Input.GetAxis("Horizontal");

    Vector3 moveDirection = new Vector3(moveX, 0, moveZ);

    moveDirection = transform.TransformDirection(moveDirection);

    if (Input.GetKey(KeyCode.LeftShift)) {
      moveDirection *= 2;
    }

    controller.Move(moveDirection * 5 * Time.deltaTime);
  }

  private void Rotate() {
    float mouseX = Input.GetAxis("Mouse X") * 250 * Time.deltaTime;

    transform.Rotate(Vector3.up, mouseX);
  }
}

As you can see, it's not that much. Let's go over it step by step – actually, there's a few things we need to do before we start coding:

1. Create a terrain by right-clicking on your hierarchy in Unity, choosing '3D object', then 'Terrain'
2. Create another 3D object that you'll place on top of the terrain. This could be a cylinder, for example: '3D object' > 'Cylinder'. Call it 'Player'.
3. Drag the main camera into the player. This way, whenever the player moves, the camera follows him
4. Add a Rigidbody component to the player and check its 'Is Kinematic' property
5. Add a Character Controller component to the player. This component comes with built-in support you'll need to control your player. Likewise leave its settings as is
6. Add a script called 'PlayerController' to the player

7. Now we can start coding. Open the script in your favorite text editor and add this public property:

   public CharacterController controller;
   

   Save your changes.

8. Back in Unity, drag the Character Controller into the controller property on your script

9. In your script, make the following change to the Update() method:

   // Update is called once per frame
   void Update()
   {
     Move();
   }
   

10. Add the corresponding private method Move():

    private void Move() {
      // When hitting 'w' or the up arrow, this will evaluate to 1.
      // When hitting 's' or the down arrow, it will evaluate to -1.
      // (To be precise, it will return a float which gradually
      // increases to 1 as you hold 'w' or gradually decreases to -1
      // as you hold 's', respectively. This makes movement smoother.
      // To get only 1 and -1, you can use `Input.GetAxisRaw` instead.)
      float moveZ = Input.GetAxis("Vertical");
    
      // Our controller will expect a vector telling it in which
      // directions to move. Here we prepare a new vector that only
      // moves along the z axis.
      Vector3 moveDirection = new Vector3(0, 0, moveZ);
    
      // We need to move along the *player's* z axis, not the world's.
      // Once rotated, your player's z axis may be different from
      // the world's.
      moveDirection = transform.TransformDirection(moveDirection);
    
      // Execute the move. We multiply by 5 since otherwise the move
      // speed is a bit slow.
      // We then multiply by `Time.deltaTime`, which is a float that
      // returns the number of seconds that have passed since the
      // previous frame was rendered. This is crucial, otherwise
      // the player would move faster on a monitor with a higher
      // frame rate as this runs once per frame!
      controller.Move(moveDirection * 5 * Time.deltaTime);
    }
    

    Four lines of code without comments. Not bad, right? Save these changes and run the project in Unity. You should be able to move forward and backward using w and s or the up and down arrow keys.

11. What about moving sideways? Easy. First, add this line below the call to Input.GetAxis:

    float moveX = Input.GetAxis("Horizontal");
    

    This works just like Input.GetAxis("Vertical") except it listens for keys a and d as well as the left and right arrow keys.

12. Change the line in which we declare moveDirection to:

    Vector3 moveDirection = new Vector3(moveX, 0, moveZ);
    

    Note that we've replaced the first 0 with moveX. You can now move sideways, and also diagonally.

13. Next, let's implement running while holding left shift. Add the following before the call to controller.Move:

    // Run while holding shift
    if (Input.GetKey(KeyCode.LeftShift)) {
      moveDirection *= 2;
    }
    

14. Lastly, we'll need to rotate the player left and right whenever we move our mouse. For this, we will add a second method called Rotate, along with a couple other changes:

    void Start()
    {
      // Lock cursor to the screen's center point and hide cursor.
      // You can get your cursor back while playing by pressing
      // the escape key.
      Cursor.lockState = CursorLockMode.Locked;
    }
    
    void Update()
    {
      Move();
      Rotate(); // <- added this line
    }
    
    private void Rotate() {
      // Determine how much we want to rotate based on the mouse's
      // movement. Multiply by 250 to make more sensitive – depending
      // on your hardware and settings you may wish to tweak this number.
      float mouseX = Input.GetAxis("Mouse X") * 250 * Time.deltaTime;
    
      // Rotate around the x axis
      transform.Rotate(Vector3.up, mouseX);
    }
    

That's it!

Known problems with this code (most of which I mentioned at the beginning of this post):

• Can't look up or down yet
• Can't jump yet
• Can 'straferun'*
• No gravity, so probably couldn't move up/down slopes

* You may notice that when you move diagonally, your character moves faster than if you move him only forward/backward or sideways. Why is that?

Vector3 moveDirection = new Vector3(moveX, 0, moveZ).normalized;
                                                     ^

The problem with this fix is that it can add a delay to your movements. You will notice that your character doesn't always stop immediately upon letting go of whatever keys you held to move. I don't know yet how to fix that and will need to review the above-linked video in which I learned about normalization.