Looking back, is there anything you would change in the language's development?

In the language design I would certainly have added functions earlier. I am rather surprised that I didn't do that as part of the original design. And the other thing I would like to have done is written a compiler for it. I got halfway through writing a shell script compiler but shelved it because nobody was complaining about performance at the time.

I can't think of things that we would have done particularly differently looking back on it. As one of the first programmable scripting languages it was making a significant impact on productivity.

If the language was written with the intention of being a scripting language, how did it become more popular as an interactive command interpreter?

It was designed to do both from the start. The design space was you are sitting at the terminal, or these days at the screen, and you're typing commands to get things done. And it was always intended that that be one of the primary functions of the shell. This is the same set of commands that you're accessing when you're in a shell script because you're (still) accessing the Unix environment but just from a script. It's different from a programming language in that you are accessing essentially the Unix commands and those capabilities either from the terminal or from the script itself. So it was originally intended to do both. I have no idea which is more popular at this point; I think there are a lot of shell scripts around.