I was watching a movie which is mostly a fun fantasy romp that bears no relation to science whatsoever but there is one scene towards the end that got me thinking; a character is sitting in a pod about 5 feet from a 200 megaton nuclear weapon, we see it count down, hear the capacitors charge and fade to white and cut to the pod exploding from the outside. Now in the scenario the character most likely has a moment to realise what's about to befall them, especially since they helped put the weapon there in the first place.
My question is if someone was sitting next to a nuclear weapon they didn't know was armed and it went off would they have time to realise they were going to die or would the criticality happen fast enough to spare them that knowledge.
I understand that gun type weapons take the longest to go critical from a standing start so I'd like to focus on them; if you were sitting next to such a weapon and heard the propellant charge go off would there be enough time to realise what was about to happen before the weapon went critical and vapourised you or would you be dead too fast to realise that the warhead next to you was live and had just gone off?