Is he interested? OR just being "professional"?

I have this co-worker I'm interested in, and over the last month, I catch him staring at me all the time now. He doesn't smile or turn away when I look at him...but one time, I did get a smile out of him when I smiled. (I'm shy, and often will break eye-contact with him.) He doesn't look at me when walking next to each other and talking, but is very attentive to what I say and what I need done...and in small, little ways, helps me out a lot. I've heard some others say he's not that nice, even though he's always nice to me. I'm not sure if maybe he's just being nice and professional, or if he's shy, and actually interested in me...