When appointing an executor, you should make sure that the person you choose is aware you would like them to act as an executor. If someone does not want to be named as an executor in your Will they can refuse. Being an executor comes with a lot of responsibility and administrative duties. You should choose people who you trust (eg family members or friends) or those with whom you have a special client relationship (eg your solicitor or accountant).
If you do not appoint any executors in your Will, the court will have to do this after your death. Once the court appoints an executor the appointed executor cannot resign or take on other executors without going back to the court to obtain permission.
It is not necessary to appoint more than one executor, although it is advisable to do so in case, for example, one of them dies before you.
If an executor dies, any other surviving executor(s) can deal with an estate. If there are no surviving executors the court can appoint one.