Jealousy is a pretty common trait in a lot of people, and even those that say they never get jealous do...they just don't like to admit it! I've always been a pretty easy going person, and don't get jealous very easily. As @Jasmin Cottontail says, a lot depends for me just what the nature of the conversation is between your partner and that 3rd person. It's one thing talking about the weather or what they are having for lunch, but if that conversation is more of a flirtatious nature then people are obviously going to become wary and wonder if there's more going on.

A lot of people say that if you trust your partner then you have no need to be jealous, and while I can see that point of view, for me it's more that I don't trust the other person rather than a partner, and there are a lot of people out there that take great pleasure in splitting couples up. They're the ones you have to watch out for, and it's those people that I don't trust.