The Guard was used only in the most dangerous moments of battles. They were the last chance, but never failed. They couldn't fail. They were the best soldiers in whole Grande Armée.
If a guard grenadier went to the battle, he had to win or die. There was no way of seeing them flee or withdraw without order. Guard grenadiers didn't use shooting much, they were tall and strong men, they prefered the bayonet attack. If they shot, they waited untill the enemy came close (about 2 metres) so the shots found their targets easily.
The grenadiers were fearless veterans, dominant power and the iron heart of the army