Aethiopica:
The complexity of the plot structuring matches the complexity itself. This is done in almost a self-consciously mocking style, vide Chariclea: "Great affairs require great preparations. Events whose beginnings the divine power has laid down in a complex way must in the end be brought to a completion in a roundabout way" (9.24-3-4), and also "if prisoners assigned to be slaves fabricate unbelievable stories and transform themselves into the king's children, as in a play".
See also the way in which Chariclea is revealed: step-by-step, with full revelation before Hydaspes not taking place until Charicles arrives "as from a stage-device, from the middle of Greece", and then confirmed by the Gymnosophist Sisimithres. This (quoting Sandy) enlarges authorial reporting by factor three, and generates and maintains suspense (see also rather gratuitous bull-wrestling match).
Heliodorus on himself:
"By divine impulse absolute contrarieties were harmonized in perfect accord" (10.38.4)
"It [their capture by the Ethiopians] was like the prelude and prologue if a play; strangers in chains who shortly before saw their slaughter balanced before their eyes were being not so much conducted as prisoners as being guarded by those who would soon be their subjects" (8.17.5)
The Greeks at Delphi "did not leave out one thing in their inquiries about Egypt, for listening to stories about Egypt is an altogether irresistible temptation for Greeks" (2.27.3)
Cnemon to Calarisis: "If anyone were to tell of the love of Theagenes and Chariclea, who is there is so flint- or steel-hearted as not to be charmed, although he listened to the story for a year?"
Calarisis: "I shall first relate my own story concisely, not (as you think) to introduce sophistical obfuscation into the account" (2.24.5). haHA!
As for duration and complexity of narration, see also Theagenes to Cnemon at 1.14.1-2; for vividness see Cnemon to Calarisis at 3.4.7
For the unreliability of narration (Calarisis, mainly), see Sandy p.40; cannot reconcile the reasons why Calarisis actually does wind up in Delphi. For the characterization of Heliodorus as omniscient, omnipotent (viz characters) narrator concerned largely with presenting a spectacle (like an emperor at the CM), see relevant bit of CCAN. Develop the implications for power relationship viz readers; self-consciousness and humour reveals possible subverting of this role (do sources support this hypothesis?). Compare with Homer's self-knowledge of the role of the bard, and its limitations