An actor I know was once called up by a casting director and told that she was wanted to play the small part of a nun. Could she travel up to Scotland from London that night? Of course, she said yes, and a page of script with a few lines for the nun arrived just before she set out for the station. The train arrived in the north, where she went straight to a hotel. Early the next morning, she was dressed as a nun, driven across wild roads and windswept moors and delivered to a field where there was a film crew. A man wandered up, vaguely greeted her and said that she was to stand on this mark, a car would drive up, and the dialogue would be delivered. As he wandered away she called after him (for he was the director), "You did want a Scottish accent, didn't you?" "Sure-Action!"
This is known as a short rehearsal period. Long rehearsals can take, well, much longer, involving months of research, living as your character would live and work-oh, all sorts of things. But the shorter rehearsal period is becoming more common, and it is therefore up to the actor-and his preparation-to give the best results on the screen.
If it is going to be your first ever piece of screen work, do not pretend you have been doing this all your life. After all, the crew will all know it is your first time, and all that energy and concentration would be better spent on learning about and observing your craft. Far better to be the known