Andy Hertzfeld was mainly involved in the development of the software end of the Macintosh story, the original Mac system software. He was responsible for many novel and innovative parts of the original Mac OS - parts we still use today, such as the ScrapBook and the Control Panel. He was also responsible for most the ROM code, and the User Interface Toolbox.
Indirectly, he also played a major role in the look of the Macintosh software, because Andy was the one who hired Susan Kare, the artist who designed most of the user interface elements of the Macintosh. The Happy Mac, Clarus the dogcow, the cloverleaf on the command key - these defining elements are all designed by Kare. You can see some of Kare's work for Apple in her portfolio. I love her work for os/2, by the way.
It's common knowledge that many of the ideas for the Macintosh were pioneered at the Xerox Palo Alto Research Center, and that Apple had access to the labs over at PARC. In the interview, Hertzfeld explains that even though there was no official relationship with Xerox, the influence from Xerox was clear - especially through Xerox people leaving Xerox in frustration over the company's focus on paper and toners, and coming to work for Apple instead. "The influence of PARC was strong in the project, but not through a formal relationship with PARC," Hertzfeld explains, "more through PARC people getting wind of what we were doing and coming to work at Apple."
Hertzfeld also explains the Apple engineers didn't have any feelings of guilt or similar feelings over using PARC's ideas. According to Hertzfeld, Steve Jobs often said "good artists copy, great artists steal". "And what that means is that when you're passionate about what you're doing you'll take ideas from anywhere and with no guilt," Hertzfeld said, "You want to make the best possible thing and that was our mentality."
Despite Steve Jobs' late involvement in the Macintosh project, Hertzfeld still credits a great deal of the success of the Mac to Jobs.
Steve made zero technical contributions. He's not a technical person, so his main contribution was setting the goal, setting a very high goal, and then being really passionate about exceeding the expectations, trying to make every single conceivable aspect of the product as brilliant and creative and wonderful as possible. So a lot of it was just the driving force and the passion. But Steve also helped in a zillion ways really, and in the organization at Apple at the time of the Macintosh already was more than 1,000 people at the company and there [were] lots of politics. The Mac kind of competed with both the LISA and the Apple II; it was sort of a computer price like the Apple II that behaved like a LISA. So as you can imagine there were lots of political pressures and the project simply couldn't have happened at all if it wasn't for the stature of Steve and the organization to withstand those pressures.
The interview ends with a discussion about Apple's focus on controlling the whole user experience, from software to hardware, and keeping all of that pretty much closed. Hertzfeld thinks that this integration and control is good for the user - but at the same time that openness and ubiquity would be much better for the platform.
As a final shameless plug, Andy Hertzfeld's book, Revolution in the Valley: The Insanely Great Story of How the Mac Was Made is now available from O'Reilly.