If you have been cursed, via Bestow Curse, and are afflicted with a 50% chance to act normally each round, what happens when to you when you are unable to act?
You simply take no action - no standard action, no move action, no swift or immediate actions, no full-round actions. It is not entirely clear whether AoOs (which aren't technically actions) are allowed, but I would rule not.
Are you considered dazed? Flat-footed?
Effectively, you're dazed - you take no actions but suffer no penalty to AC. Technically, you're not
actually dazed, so if there were any spells/powers/effects that only work on a dazed creature they wouldn't affect you... but I'm not aware of any such spells/powers/effects. (And, FWIW, if faced with that, I would rule that they do work, on the grounds that "if it looks like a duck...")
What if you concentrating on a spell? Does it fizzle out?
Concentrating on a spell requires a standard action, so you can no longer do that. Note that some spells have a duration of "Concentration + X rounds", in which case the spell will last for a further X rounds.
What if you are being overrun, grappled or tripped?
For grappling, you still get the opposed grapple checks to defend yourself. But you can take any actions to escape the grapple, damage an opponent, or similar.
Trip works normally. Note that, if tripped, the character can't take the required action to stand up.
Likewise, overrun works normally - including the ability of the character to simply let the attacker go past.
