if you are interested you (the guy) will flirt a lot and stuff, although if you both like each other it'll probably go beyond flirting...i am shy but if i know a girl likes me i become pretty confident with them
if you dont like the girl it depends, sometimes it makes things awkward, it depends on how the girl is and reacts to finding out you dont like her i think, if they continue to act the same and dont make a big deal about it it shouldnt be a problem and you can continue being friends...although how she (the girl) reacts will probably depend on how much she likes you and whether she would actually want to be your friend if not being your girlfriend.