The purpose of the rehearsal is so everyone can be prepared for going on location to film the actual scene. Quite often the sound, camera and lighting crew will be present so they can plan for their particular involvement.
It is important that you arrive knowing your lines so that the director can give you the moves that he wants to go with them. This is called blocking and happens in all film and theatre productions. Once the director is satisfied with the movements he will begin to help you deliver the individual lines to fit the script as he sees it.
You will also rehearse your body movements as they need to be at the same time, every time. For example drinking a cup of tea, must always be done with the same hand and at the same part of the line otherwise when it is edited the cup will appear and disappear in the wrong place.
If it is possible you will have at least one rehearsal on location so that any problems can be ironed out and everyone feels comfortable when it comes to the day.
Rehearsals are really important and it is vital that you attend all of your rehearsals. It is difficult to rehearse one side of a conversation. If you can't make it for any reason you need to give plenty of notice.
Most of the time the directors will allow you to wear something from our own wardrobe, but on occasion, you may need special items which need to be discussed.