People seem to marvel at Philip Glass' prolificacy. A couple of years ago I was privy to see a draft biography for Glass for a prominent Dictionary of music which included mention of his "suspicious" output and that it was well known that he employs a team of young composers variously composing and arranging his music for him. The whole thing was libelous. Nothing could be further from the truth and this type of thing usually springs from either jealous colleagues or ill-informed journalists who simply don't like his music.
In any case, witnessing Glass working and delivering manuscript pages day in and day out is a testament to how much one can accomplish over a lifetime, piece by piece. Furthermore, Glass isn't nearly as prolific as say a Mozart or Schubert or Hovhaness or Brian to cite 20th century examples. To put a fine point on it, every Glass piece that anyone has ever heard was hand composed, arranged, and orchestrated by Philip Glass himself. It is the product of pencil, paper, hard work, and imagination. Any exceptions, like Michael Riesman or Paul Barnes transcribing or arranging Glass' music for solo piano, are credited as such.
There's also an element of the hand-made in the Glass process. Honegger used to equate writing music, in the age before synthesiezers, to ship-building. One didn't know before plopping the ship into the water whether it would capsize or sink. Such it was with composition: a composer couldn't really know what a piece sounded like before hearing it played. Up to the point of the first rehearsal the piece only existed in his mind's ear. This is still Glass' process.
Writing music by hand on paper is almost a totally lost art. It's totally understandable why no one does it anymore. It's like writing a book long-hand. The biggest difference is that when one writes by hand, there's a commitment to the ideas before you write it down for no other reason that it takes so long to copy a page of music by hand. When writing with a computer, a whole piece can be transposed into a different key with the click of a mouse. Such things are impossible the way the old composers used to do it. Glass writes usually two drafts before a "Fair Copy" finished score. Imagine how long it takes to copy an opera of 200 pages fully orchestrated length.
So it's a lot of work but one is constantly working the "inner ear" and coming up with good ideas before writing them down. The "hand-made" quality of the music comes through.
With that said, Glass' constant stream of productivity is also the product of timing. Often times the pieces that are premiering were written long before. Symphony No.9, for example was written almost a full year before its premiere. The composer had two big opera commissions looming (The Perfect American, The Lost), premiering in 2013 and so almost the entirety of 2012 was spent on those. And in fact, The Lost (which premiered last month), was composed before The Perfect American which premiered in January.
This is all about process and time management. With all that said, sometimes things are premiered soon after the composition. This is the case with a new piece TWO MOVEMENTS FOR FOUR PIANOS which will have its premiere next month in Ruhr. The Piano Festival Ruhr was the commisisoner and site of the premiere of FOUR MOVEMENTS FOR TWO PIANOS five years ago. When one thinks about the timing, that means this is the first piece to be written by Glass since The Perfect American which premiered back in January, the composition of which finished probably a couple of months before that.