"Resident Evil Zero" was designed to be more difficult than its predecessors, removing the item boxes to make the game more like "Sweet Home" (1989). The real-time "partner zapping" system was designed to take advantage of the console's unique features and strengths, namely the lack of load times, which are necessary for optical disc based gameplay as with the PlayStation. In an effort to make 1-on-1 zombie fights more intense, Capcom experimented with giving the zombies different reactions when they were shot and allowing the player to counter-attack when bitten. The team also toyed with faster zombies, a precursor to "Resident Evil"s crimson head enemies. The prototype also supported local co-operative play. "Resident Evil Zero" was officially announced at 20 percent completion in January 2000, after which it was presented with a playable demo at Tokyo Game Show. The game was expected to release in July 2000 and reportedly had an atmosphere close to the first "Resident Evil", focusing more on suspense than the more action-oriented gameplay of "Resident Evil 2" (1998). However, development began to slow down when it became apparent that the data for "Resident Evil Zero" would not fit on a single cartridge.