Microsoft explains how Cortana was created and what makes her so different from Google Now and Android
"We focus a little bit more on contextual triggers that we think people will actually understand. We always talk about 3 triggers—we talk about time, we talk about location and we talk about people. Those are the triggers people get. Let’s just focus on a set of things that are going to be of high utility and limit the number of triggers so that people can understand what the system is capable of.
For us is a notion of personality. When we look at Google, they’ve made some pretty clear decisions. It’s about getting you quickly and efficiently to Google’s services. It’s not about personality. There’s just something really delightful that makes people smile about having an anthropomorphic personality inside this assistant. We studied this a lot and looked at people’s reaction in labs; it just makes people smile. It also opens up this type of trust relationship we talk a lot about.
Google has got a decision to make around how they’re going to create a personality"