But he talked about two things the most.
1. Develop your gift. You either have the gift to compose music or you don't. It cannot be taught. The musical language can be taught, but the genius to create music is a gift. So, do the work to find out if you have the gift.
2. Be true to your gift. Develop a solid core of belief in yourself no matter the criticism. Your music might not be in the popular style of the day or be hailed by the masses. But it needs to be truthful to you. Even if there is criticism, genius will always find an audience.
He encourages new composers to do everything in their power to get their music performed, even if it is just rounding up friends for a read-through. Nowadays, composers use notation programs that can playback their scores with amazing fidelity. They can even record their music by themselves through the miracle of multitrack recording. (No friends required, but it's seriously more fun in a group.)
John Rutter says that although he enjoys writing for orchestra, he always feels that when he writes for instruments, he is just a guest at the table. But when he writes choral music, he is home. He encourages all composers to write what feels like home to them.
"Where do you get your inspiration?" He says that all composers are standing on the shoulders of giants. There is no shame in borrowing (or even stealing) ideas from the master composers to get started.
Wow! I feel better now.