I think it’s now 100% confirmed that person doing the flip in the first OP was Jean – the choice decision? According to the director, “the candidate that represents humans” – a recurring comment n’est pas?
Scene from the BD/DVD volume 2 omake videos – ちみキャラ劇場 “とんでけ！訓練兵団”. He joins the trio in their titan stomach “adventure” this time (or just Mikasa and Eren?). Jean’s worst nightmare comes true! Well, from the looks of the picture, maybe not a total nightmare. 😛