At some point, octave passages appear out of nowhere and one is supposed to play them.
Oftentimes, "octave newbies" approach their playing so stiffly that they get sore arm muscles etc.

Obviously, this is something to be avoided.

In a separate discussion of finger usage, I talk about proper finger usage when playing single notes.
Playing octaves (or other simultaneous two note intervals) really should be no different.

To this end, practice playing octaves:
*By pressing the thumb first, and then adding the 4th or 5th finger (to give attention to both fingers). Observe that the fingers are not "locked into a claw" when you are doing that.
*Vice-versa. Play finger 4/5 first, then add thumb.
*Work on minimizing the time gap between pressing of the two notes, without losing the sensation of individual finger bending/contracting actions as they slide on the edges of the keys.
*Practice this quietly, without stiffening the large arm muscles