I can't speak for pilots but for cabin crew you would need to have cabin crew experience.
Generally, airlines will appoint trainers from within cabin crew ranks. There is usually a lot of competition as these positions are quite sought after.
Airlines that I have worked for have had full time trainers who were once crew with the airline but now only train. They are supported by people who will say, train one month and fly the next. Some may fly just once in a while to keep up their license.
At an airline I flew for years, I did 2 days a month in recruitment and a day in the training rig with abinitio and recurrent cabin and flight crew. I might do a day of admin and the rest of the month would be taken up by mostly long haul flying. My instructor training was conducted in-house and was given to me because I had hand-on experience involving a couple of SEP related incidents.
BA have had trainers that were experienced cabin crew/trainers but have left the airline altogether and are re-employed as contractors.