This is a common misconception perpetrated in media that a seizure must be a convulsion (the kind where the person loses complete control of their body). Thus, epileptic characters will always have these seizures. In fact, there are many different types of seizures that have symptoms that go from simply losing consciousness and staring into space for a few seconds to losing control of a single limb but maintaining consciousness, to the full-blown writhing on the ground episodes. This trope probably exists due to Rule of Drama, as well as a concern that the audience won't be able to tell that someone is having a seizure unless they're convulsing. After that original idea has taken hold, The Coconut Effect ensures its continued use.