From The History of Philosophy, from the Beginning to Plato:

If this is so, then we need to steer a middle course: neither should we assume that Plato takes literally all the many ideas that he develops through his characters in the dialogues (which would be dangerous on any account), nor should we attempt to eliminate altogether what may seem to us the more fantastic and apparently poetic elements among them. (Indeed, for some Neoplatonist and Renaissance interpreters the latter probably take us closer to the core of Platonism.) We must remain aware that Plato’s philosophical writing is a complex matter, and that his motives as a writer may sometimes directly affect the content of that writing, as indeed may his chosen literary form. Thus, for instance, particular dialogues will often follow out a particular line of thought to the exclusion of others, which it is difficult to bring in within the fiction of a particular conversation (the treatment of immortality in the Symposium is one clear example; see above).