Apparently they were roommates when they both were attending Juilliard. Here's a particularly touching (and typically hilarious) quote from Reeve's autobiography regarding a visit from Robin at the hospital:

"Then, at an especially bleak moment, the door flew open and in hurried a squat fellow with a blue scrub hat and a yellow surgical gown and glasses, speaking in a Russian accent. He announced that he was my proctologist, and that he had to examine me immediately...it was Robin Williams...for the first time since the accident, I laughed. My old friend had helped me know that somehow I was going to be okay."