I'm not sure of the training the crew go through but I believe they're of the type where it talks you through each and every step of the process, should it be needed
Automated External Defibrillators (AED) do indeed give you explicit instructions as to what to do; the ones you see in public places are for
anyone to use in a cardiac arrest scenario. I never used an AED when I was in the ambulance service (they are carried but just to get around the requirement to carry a defib to every patient's side from a elf n safety point of view, the manual machines are heavy) just the manual machines for professionals as you'd expect, many hundreds of times.
The first time I used AED was when mid-way through a First Aid course I was teaching at a golf course... not on a delegate but an elderly member of the club. So having never seen the machine, as we teach in First Aid, I switched it on and did as I was told! Sadly we didn't get the outcome we wanted, but not for not trying.
If someone is collapsed and not breathing and a public access AED is nearby please get it involved, if its in a case the 999 operator will release the keysafe code to you. Do as you're told by the machine and there won't be anything to worry about from a legal point of view. Every minute defibrillation is delayed the survival chance decreases by 10%.