Runtime definition (and redefinition) of functions was critical to the Lisp Machine's usage model. Since the entire environment was a single Lisp session, everything happened at "runtime", including defining functions. Most development, then, was done by interactively defining functions and trying them out. It certainly was possible to compile a source file into a binary file, and there were occasions when one had munged one's Lisp session to the point that rebooting and reloading the binaries was required, though those were relatively rare.