What's the difference between Update and FixedUpdate in Unity, and should I bother?

Question

I was told to use Update() for graphics and FixedUpdate() for physics, but sometimes my inputs won't register when I use FixedUpdate(). It works fine if I use Update() for everything though.

Should I bother with FixedUpdate(), or am I doing something wrong?

Answer 1

FixedUpdate can run faster or slower than Update depending on your settings. That'll depend on load (how fast Update is running) and speed you have set for FixedUpdate (found in Edit->Project Settings->Time). Ideally, since Update is run once per frame, this is where you want to capture input. If you need to act on input in the FixedUpdate method (like controlling physics bodies), set flags in Update then handle them in FixedUpdate:

bool leftMouseDown = false;

void Update()
{
    if(Input.GetMouseButtonDown(0))
        leftMouseDown = true;
}

void FixedUpdate()
{
    if(leftMouseDown )
    {
        //update physics bodies with input
        leftMouseDown = false;
    }
}

It's still a good idea to control physics from the FixedUpdate method, since it's unlikely the physics needs to be updated as often, and physics on a fixed update is much easier to predict (determinism) than physics on a variable update.

Answer 2

You need to understand what each one does. Update() gets called as often as possible (not sure, maybe it can be capped), either way - each frame. FixedUpdate() gets called every constant amount of time (hence "fixed").

Input goes into Update(), as simple as that (because as you noticed FixedUpdate() might not catch the input event). Game logic however might go into either one. Physics needs to be deterministic and that's why it should be in FixedUpdate(). Other things don't have to. Depending on the usage you have to decide which function is appropriate.

Althouth it's worth to say that some applications have all game logic update in FixedUpdate() (even outside of Unity) - it's called "fixed step game loop". Having all update code in a fixed step function gives you determinism and makes your app more likely to behave the same way each time (and on each device). Also allows for some fancy features, like replays.