If you're a guy, its really not the best idea to act too differently around different girls. Their friends will invariably notice, and they'll be tormented about it (great if they like you, not so much if they don't.) I speak from experience..!
If you're a girl, debating whether a lad likes you, hmm.. I hate it when lads aren't clear whether they just want friendship, or more.. so good luck! xxxx
EDIT: and yeah, I know I'm not a guy, so you might as well just ignore the above if you're trying to gain a deeper understanding of the male psyche! xx