Here's a bit from Founders at Work.
Livingston: How did you know Trevor?
Graham: Trevor was in grad school with Robert. I asked Robert, "Who's the smartest grad student in the computer science program?" and he said "Trevor." I couldn't believe it actually, because at the time I thought Trevor was a total goofball.
Livingston: But you were soon convinced he was talented?
Graham: Trevor is a prodigy, in the original sense of the word. When we first recruited him, we asked him to write this little piece of image-manipulating software, to kind of test him out. For 2 weeks we heard nothing from him, and I had pretty much written him off. Finally he sent me an email asking me to come to his office to see what he'd done. I went there expecting to see this new image software, and instead he's rewritten our entire system in Smalltalk— everything I wrote, plus everything Rtm wrote.
I basically said, "OK, you're hired. Now go and write the damn image software, because we're not rewriting everything in Smalltalk."