Drunk in his Titan form.

It’s been a rough, grueling day of harsh sweaty conditions of training as a trainee. As the evening nears, Reiner suggests just you and him head out to town and sneak yourself a drink. You agree, but later the soldier gets a bit too drunk on all the rounds, a whole lot less cautious. He drags you out of the village and to the woods where he can drunkenly reveal his Armored Titan form to you.