Though both need actors, immersive games are very different to theatre. In auditions, we're evaluating people on three axes:
a) Character Fit
We have a sense of the character in our heads already. For example, in Shadow Over Southwark, Lord Mayhew's brief is:
"Genuine action hero. Decisive, capable, charismatic, and fearless. The kind of person you'd follow into battle. Proxy for the players.
Ideally 35+, tall, muscular. Any gender.
Mayhew loves London and all the people of London - rich, poor, and in between - and s/he will defend them to the death. He’s Henry V rallying the troops (he knows it too, he will happily quote some Shakespeare), total Alpha, can wither someone from a distance with a stare. He is a protector and will put everyone ahead of himself, but will sacrifice a friend for the greater good. He once faced off a whale in the Arctic circle, huddled with a pride of lions during a cold night in Kenya, and has a lover in every Borough because he also possesses Bond-like charm. He needs, nay, deserves, a catchphrase. Will flirt with all players."
Can this actor realistically portray this character, both physically and in terms of personality?
Regardless of the particular role, is this actor skilled? Can they project, speak clearly, move well, respond to direction, and handle improvisation?
Can we rely on this person? Do they arrive on time (or tell us if they can't), know their lines (or tell us if they don't), and work well with the often-stressed stage manager? Do they remember details? Can they handle the pressure?
Where we're different to theatre is that we will absolutely sacrifice (A) in order to get (C) and (B). The most important thing about an immersive game is this: it has to work. Being out in the real world with players who have genuine agency already introduces so much uncertainty into the system that there's no room for any more. We are building a relatively small, tight, trusted crew of people who can handle anything that we throw at them, and that goes for our actors as well as our infrastructure team.
In our general hiring, we start with a Job Spec like everyone else - but once we find amazing people, we absolutely mutate the spec to fit what they can, and want, to do. The same goes for actors - we find first-rate people and build roles around them.
And if that means we have to rewrite the script, so be it.