Reading the encounter between the crew of The Vigilant and Cthulhu, while most of the crew were killed by violence, died of fright, or went hopelessly insane, Johansen was able to act rationally. He went into shock after the encounter, but he retained his sanity, albeit with horrible memories of the encounter. So I think there is a case for allowing a small chance for players to somehow hang onto their senses no matter how horrific the encounter. http://www.dagonbytes.com/thelibrary/lovecraft/thecallofcthulhu.htm
Three men were swept up by the flabby claws before anybody turned. God rest them, if there be any rest in the universe. They were Donovan, Guerrera, and Angstrom. Parker slipped as the other three were plunging frenziedly over endless vistas of green-crusted rock to the boat, and Johansen swears he was swallowed up by an angle of masonry which shouldn't have been there; an angle which was acute, but behaved as if it were obtuse. So only Briden and Johansen reached the boat, and pulled desperately for the Alert as the mountainous monstrosity flopped down the slimy stones and hesitated, floundering at the edge of the water.
But Johansen had not given out yet. Knowing that the Thing could surely overtake the Alert until steam was fully up, he resolved on a desperate chance; and, setting the engine for full speed, ran lightning-like on deck and reversed the wheel. There was a mighty eddying and foaming in the noisome brine, and as the steam mounted higher and higher the brave Norwegian drove his vessel head on against the pursuing jelly which rose above the unclean froth like the stern of a daemon galleon. The awful squid-head with writhing feelers came nearly up to the bowsprit of the sturdy yacht, but johansen drove on relentlessly. There was a bursting as of an exploding bladder, a slushy nastiness as of a cloven sunfish, a stench as of a thousand opened graves, and a sound that the chronicler could not put on paper. For an instant the ship was befouled by an acrid and blinding green cloud, and then there was only a venomous seething astern; where - God in heaven! - the scattered plasticity of that nameless sky-spawn was nebulously recombining in its hateful original form, whilst its distance widened every second as the Alert gained impetus from its mounting steam.