I find it best to be in character as soon as you open the door.  You would ask for a "hard copy" of the presentation at the beginning, i.e your payment in a folder and roll (role) with it.

Unless you have a really skilled role player - few and far between - they tend to drop character after about 10mins! I continue even after they have drop character until it is obviosu that they are not bothered, as sometimes they come back into role play.

Have fun x

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As La-lique says you can often incorporate payment into the roleplay. For example one client I see enjoys being a bailif and I am the innocent naive 20 something who got behind on her repayments.. what ever will I do? Of course, he suggests we come to some arrangement which my character is not overly keen on doing but she soon gives in to him .

When he comes in the first thing he does is hand me an envelope, he then tells me it's a letter for the seizure of goods and the outstanding amount has just gone up for him having to deliver the letter in person (and variations to that effect). He tells me to fetch him a drink and have a read of the letter in private while he makes himself comfortable before we have a 'little chat' (this is my cue to check the money in the envelope and put it away).

Go with the flow and try to have fun with it but do make sure there is some kind of safeword. 

All went well, he asked me to play incompetent nurse...which really wasn't a stretch for moi. Had powerpoint with role play script and, punishments etc. Was grand actually, didn't have to think - always a plus. He did take out rope at one stage though, which I wasn't expecting. Kinda freaked me out, surely is something that should be discussed before hand?