Except I would see no reason for Brando to even be on the premises...since this was a filming of a scene from a movie in which he did not even appear.

I would guess the lock fumbling was purposely staged and not ad-libbed, since the Genco character casually walks over to assist & has no problem opening the door. The whole thing aids in having Signor Roberto look even more foolish as he hopelessly tries to exit Vito's office with some small degree of dignity.

One joke Brando did play during The Godfather was to add weights to his stretcher for the scene where Vito is being brought up the stairs upon returning home from the hospital.

Apple