Hey its not written anywhere that you have to best friends with your co-star............though generally, people who share good rapport off-screen will sizzle on-screen too! But I think Kareena Kapoor and Shahid Kapur are a good example of off-screen couple falling flat on-screen but then most of the spectacular on-screen combos have been made by peope who share a good relationship off-screen as well....SRK - Kajol / SRK - Rani / Aamir - Juhi / Saif - Preity / Saif - Rani / SRK - Preity / Abhishek - Rani and so many more!
But I guess Iqbal and Neha are an exception! They might not be too fond of each other in person, but none of that is shown on screen! They look pretty good together!
Chilly