From what I've heard, the first one was the only scary one. The rest were more of action games than horror games.
English
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From what you hear is based on Dead Space fans who played the original extensively and their familiarity playing as Isaac aka "The necromorph slayer" subsided their fear since they knew how to deal with necromorphs and the player knows Isaac is used to it as well. Most people who play 2 & 3 without playing any other usually get just as scared as people had in the first. Familiarity helps subside fear.
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You heard wrong. Dead Space 2 just had much more player dialogue. Those are the main changes making it "less scary" in a way. Both games are incredible nonetheless and I would recommend you buy one.