When I was a beginner I'd write code and run it very fast, after each small change, even though an experienced developer could probably compile it in their head

When I was a beginner I'd write code and run it very fast, after each small change, even though an experienced developer could probably compile it in their head and see whether it worked or not. I think just as you get more experience you tend to write more code before you run it. I've written an entire (relatively small) application and written the tests for it before I even thought about running it. Run it, few tests fail, iterate a tiny bit, done. It's just natural as you get more experienced that you can reason about code in your head without resorting to running it to confirm it.