If the person you are after is not looking for a relationship right now, you can bet they won't change their mind for you. Before you get your heart set on pursuing something with this friend, make sure they are open to a relationship and know what they are looking for in a mate. Did they just go through a bad breakup? Are they graduating from school and moving 200 miles away? Timing is everything, and sometimes things just aren't meant to be. It's also important to evaluate your own feelings. Are you willing to risk your friendship to take things further? If you just want to see what it would be like to bang this person, quit while you're ahead.