This is Maugham in 1907, on how he came to write Lady Frederick, the first of more than thirty plays to be (as almost all of them were) a huge success: 

I reflected on the qualities managers demanded in a play: evidently a comedy, for the public wished to laugh; with as much drama as it would carry, for the public liked a thrill; with a little sentiment, for the public liked to feel good; and a happy ending. Having made up my mind upon this the rest was easy.

And there, early in an astounding career which made him more famous than any writer could hope to be today, you have much of what he was as a writer. 

But not all. It leaves out his consuming interest in people – the detached but piercing interest of a naturalist studying a species – and his creative energy. He was not a great writer, but he was a phenomenon: a living machine compelled by its nature to observe, and then to turn what he had observed into stories (or plays, or novels) which people enjoyed so much that they sold by the million all over