The high-level design of the companions and areas in F:NV (outside of the Strip) was entirely mine, as was the entirety of the system design, but I did not do any of the hands-on design for any quest or location, I didn't design the central plot or conflict (that was John Gonzalez), and I only wrote a small number of dialogues (Arcade, Hanlon, Kimball's speech and a few others).
I dictated the overall tone and direction of the setting, story, and system mechanics, but there was a huge team of people responsible for doing the actual design work and implementation. At times, I demanded specific things, like the ability to kill any non-child NPC in the game and still complete the critical path. In most cases, I let the designers have a lot of freedom within a loose outline of what an area was supposed to be. Sometimes the designers convinced me to change the outline of those areas, so in the end, they had more influence over the high-level design than I did.