Benjamin Barker alias Sweeney Todd, is a bitter man, out for revenge. Todd's main musical motive is indicative of this: the inversion of the Dies Iræ motive. And there is more: the part of Todd is written for a bass-voice (which is used mainly in the lower registers). We tend to associate low notes with grief, sadness etc... All things Todd feels.
The fact that Todd is using an alias is also portrayed in the music. For instance, if we examine Pirelli's Miracle Elixir, most of Todd's utterances are in a higher register and void of his personal motive. But as soon as he drops the ``mask'' in the Epiphany all the principals come back: lower register and personal motive.
A further investigation of the linguistic characterization of Todd reveals that he uses iambic meter5.13. What strikes me as odd is that Todd does not speak Cockney, which what we could expect had he lived in the vicinity of Fleet Street. In my opinion this would have enhanced the character of Todd, but also to the portrayal of the locale of the action.
Another aspect of Todd's linguistic repertory is that his sentences are short and at times no more than exclamations. Furthermore, the speech rhythm is very coarse: ``There's a hole in the world like a great black pit and the vermin of the world inhabit it, and it goes by the name of London''. The rhythm is very accentuated and sharp.
When Todd thinks of Lucy or Johanna however, his rhythm becomes more fluid. The melody suddenly seems to be based on descending semi-tones, which lend it a more seductive tone.
This inner division makes Todd a `tragic man' who is essentially `divided'. It can be argued that the Epiphany is an exposition of this essential part of Todd's character. Therefore, it therefore has a special position in the plot and even though it occurs near the end of Act I, it is still a part of the exposition.