My recommendation would be to stop watching it about half way through and make your own ending up because I guarantee it'll be more clever and satisfying than the film's. The twists aren't clever nor believable. The characters have no depth and do not develop much and the little that they do is abruptly binned in a ridiculous twist, which leaves you wondering why whole scenes were filmed at all.

I have no idea why such big stars put their names and faces to this - I hope this isn't the film they're remembered for. Unbelievably, I've just discovered that they made some sequels. I just wish the Hollywood writers' strike had happened sooner.