Matt Nix went through the auditioning process for children to play the character, Jack, but ultimately decided that rather than acting chops, the rapport with his son, Charlie, would be the most beneficial.
The backwards and kind of "opposite day" feel that this episode works through was deliberate effort put in by the writer, creator, producer, and director, Matt Nix. Watch for the pacing to be off, the problem solving to come differently, and some substantial "Left field" character development.
Carla: We recruited you because you're a good liar. I guess we can't complain too much when you lie to us.
(On the phone with Michael)
Fi: Yeah, I'll be there in a bit. We're playing army men.
(continues playing with Jack)