I just learned that an adverb or quality describing how a verb (action) is performed must be preceded by the filler word 得, which introduces an "expression of degree" ("well", "badly", "quickly", etc.).
But, while 得 occurs immediately after the main verb, it does not necessarily precede the descriptive quality directly. If the quality describing the first verb is conveyed by a descriptive verb (or adjective-verb), it seems that 很 must always precede the descriptive verb in simple affirmative sentences, provided that no additional emphasis of the quality is given. As such, if I understood correctly:
她说得好吗? - Does she speak well? (interrogative sentence, no 很)
她不说得好。- She does not speak well. (negative sentence, no 很)
她说得那么好。- She speaks so well. (no 很 here, since 那么 replaces it as the emphasis)
她说得很好。- She speaks well. (simple statement with minimal description, 很 required)