All of us are capable of being zealous, regardless of our personality type. Even the most phlegmatic person can be passionate about something. Some people are into soccer, for others it's food. So the issue isn't whether or not one can be zealous, but where a person's devotion lies.
In Romans 12, the apostle Paul provides some instruction that includes being zealous in serving the Lord. John Piper paraphrased it this way: "Do lots of work for Christ, passionately". The words "never be lazy, but work hard. . . enthusiastically" (v. 11) emphasizes being earnest and devoted in getting things done. What does that look like? Consider the following questions: Are we efficient, or do we procrastinate?; Do we work hard to get things done, or do we give up halfway?
Romans 12:11 also contains this idea: Do lots of work for the Lord passionately - not being grouchy. A person passionate about serving the Lord doesn't consider the number of hours he has clocked in for the Lord. He's willing to do more, and he does it without complaint or protest. The reason is simple. Serving Jesus is the highest privilege in the universe for human beings. In view of God's mercy, let's strive to be zealous Christians - passionately serving Jesus.