How does the Arcane Thief work?

[IncHulkFan]

I have a player that is playing an arcane thief and I want to make sure that we play it correctly. My question has to do with the Steal Spell ability and how it works. If the arcane thief has a higher initiative do they say they are waiting to try to steal a spell from a caster with a lower initiative or do they try to steal the spell and then not get to do anything the next round? If the opponent has a higher initiative can the arcane thief try to steal that spell even though the opponent got to act first?