There will be about forty or fifty of you for the week. Names are drawn from a hat at the start of each case. If called, then as you walk forward, either counsel may say "no" without any reason, and you go back (until the next case). Each counsel is allowed three "noes". Dressing to look rather upper-crust increases the chance of defence counsel using one of his "noes" on you.
After the case starts, the judge tells all those who have not been called when to come back to be ready for the next case. Maybe after lunch, maybe tomorrow morning, maybe three days, whatever.
I've been on duty twice, and called to the jury once.