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 th...

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. @Lewis). The diplopia ought to be binocular (indicating dysconjugation of the eyes) instead of a monocular (usually refrac...