Beyond fatigability, what distinctive features of diplopia characterize myasthenia gravis?

The key to ocular myasthenia is variability- maybe more than fatigue. If the diplopia is always in the same direction and even more clearly stemming from one eye (and muscle) then this is more likely to be an isolated muscle weakness- whether from nerve disorder or latent strabismus. But if the diplopia is sometimes horizontal, sometimes skew, sometimes vertical, sometimes to the left and sometimes to the right- MG is more likely. Finding abnormalities in both eyes with variability or fatigue also makes MG more likely. Having other symptoms and signs other than diplopia also helps.

Myasthenic diplopia is often characterized as an ophthalmoparesis that is fatigable (as described in the prompt of the question), but also variable (as was well-described by Dr. @Dr. First Last). The diplopia ought to be binocular (indicating dysconjugation of the eyes) instead of a monocular (usually refractory) disturbance. Another curious thing about ocular myasthenic diplopia is that it might mimic other opthalmoparetic patterns, including cranial mononeuropathies, or even brainstem pathologies (mimicking a classic internuclear ophthalmoplegia for example) but without causative structural abnormalities on imaging of the brain/brainstem. Lack of structural disturbance, with fatigable and variable nature, and lack of other clinical patter...