Start and Update methods in Unity - Step-by-Step Execution

Concept Flow - Start and Update methods

Game Object Created

↓

Start Method Called Once

↓

Game Loop Begins

↓

Update Method Called Every Frame

↓

Repeat Update Each Frame Until Game Ends

When a game object appears, Start runs once to set up. Then Update runs every frame to keep things moving.

Execution Sample

Unity

void Start() {
    Debug.Log("Game Started");
}

void Update() {
    Debug.Log("Frame Updated");
}

This code prints a message once at start, then prints a message every frame.

Execution Table

Step	Method Called	Action	Output
1	Start	Runs once when object is created	"Game Started" printed
2	Update	Runs first frame update	"Frame Updated" printed
3	Update	Runs second frame update	"Frame Updated" printed
4	Update	Runs third frame update	"Frame Updated" printed
...	Update	Runs every frame after	"Frame Updated" printed each time
Exit	Game Ends	Update stops running	No more output

💡 Game ends or object destroyed, so Update no longer runs

Variable Tracker

Variable	Start	After Frame 1	After Frame 2	After Frame 3	...final
Game State	Initialized	Running	Running	Running	Running until game ends

Key Moments - 2 Insights

Why does Start run only once but Update runs many times?

Start runs once when the object is created to set things up (see execution_table step 1). Update runs every frame to keep the game active (steps 2 and after).

What happens if you put code in Start that you want to run every frame?

It will only run once at the beginning, so the game won't update continuously. You need to put repeating code in Update (see execution_table steps 2+).

Visual Quiz - 3 Questions

Look at the execution table, what is printed at step 1?

A "Frame Updated"

B "Game Started"

C Nothing

D "Game Ended"

💡 Hint

Check the output column for step 1 in the execution_table.

At which step does Update first run?

A Step 3

B Step 1

C Step 2

D Exit

💡 Hint

Look at the Method Called column in the execution_table.

If the game ends after frame 3, what happens to Update calls?

A Update stops running

B Start runs again

C Update runs again at step 4

D Update runs twice at step 3

💡 Hint

See the exit_note and last row in execution_table.

Concept Snapshot

Start() runs once when the object is created to initialize.
Update() runs every frame to update game logic.
Start is for setup; Update is for ongoing actions.
Update keeps running until the game or object ends.
Put repeating code in Update, not Start.

Full Transcript

In Unity, the Start method runs once when a game object is created. It is used to set up initial states or variables. After that, the Update method runs every frame, which means it repeats many times per second to keep the game active and responsive. The Start method is like turning on a machine once, while Update is like the machine running continuously. If you put code that should repeat inside Start, it will only run once, so the game won't update properly. The Update method keeps running until the game ends or the object is destroyed.