Beethoven was revising Fidelio when he wrote to Georg Freiedrich Treitschke (who was helping to revise the libretto) (1):

Now, of course, everything has to be done at once; and I could composer something new far more quickly than patch up the old with something new, as I am now doing. For my custom when I am composing even instrumental music is always to keep the whole in view – But in this case the whole of my work is – to a certain extent – scattered in all directions; and I have to think out the entire work again – To produce the opera in a fortnight is certainly out of the question. I am still convinced that it will take us four weeks. Meanwhile the first act will be finished in a few days – But there is still much to be done to the second act and I have to compose a new overture as well; but this is the easiest task of all, I admit, because I can write an entirely new on – Before my concert I had just made a few sketches here and there, both in the first and second acts; and only a few days ago I was able to begin to work them out – The score of the opera has been copied as wretchedly as anything I have ever seen; I have to check every single note (it was probably stolen).  In short, I assure you, dear T[reitschke], that this opera will win for me a martyr”s crown. Had you not taken so much trouble with it and revised everything so satisfactorily, for which I shall ever be grateful to you, I would hardly bring myself to do my share – But by your work you have salvaged a few good bits of a ship that was wrecked and stranded.

Anderson, Emily, ed. (1961) The Letters of Beethoven. 3 Vol., London: St. Martin”s Press. Letter 479. Cited in: Marek, George (1969) Beethoven: Biography of a Genius. London: William Kimber, p.471-2.

(1) The original German libretto was by Joseph Sonnleithner from the French of Jean-Nicolas Bouilly.