Old version for comparison:
func _fixed_process(delta):
var is_moving = Input.is_action_pressed("move_up") or Input.is_action_pressed("move_right") or Input.is_action_pressed("move_down") or Input.is_action_pressed("move_left")
direction = Vector2()
if is_moving:
speed = MAX_SPEED
if Input.is_action_pressed("move_up"):