In SMG, the second player collects star bits and occasionally presses A to hold onto most enemies or hazards to stop them in their tracks, like some boulders and bullet bills. They can press B to shoot star bits and stun enemies, but I can only recall one time I actually needed to perform this function.
RE:UC is a great transition because it's a light gun game, you just point the wiimote, shoot with B. Of course, in RE:UC you share a life bar so the second player needs to do their part of the defensive gestural duties. Like the first player they can swing their knife, and like the first player they will occasionally be grabbed by enemies and must execute a waggle to counter-attack instead of getting hit. Certain sections of each level include Quick-time-event challenges where both players must execute different gestures or button presses to avoid an obstacle, and if just one person fails their command (pressing A repeatedly, for example, or waggling) then you get hit.
You could always look to the other Light Gun game, Ghost Squad, to help ease in "second players" though Ghost Squad IS yet another shooting game and derives some of its later bonus mode humor from being arcade-like-wacky (the paradise bonus mode uses water guns, women in bikinis, and bananas as knives).
But a real GREAT possibility to ease another player in is Trauma Center New Blood. Everyone understands playing doctor, and this one features co-op extensively!