According to the Puppetmaster series, there is a recipe for ‘animating dead tissue’ in the Necronomicon, which has been discovered and developed by a Nazi Scientist in WW2. And it looks like -lo and behold- Reagent? Whose theories did Herbert West study? The ultimate crossover of all the things.
In the scene where Bruce was first hit by Ellen Sandweiss while she acted out her scene as deadite Cheryl attacking, the bookshelf wasn’t originally supposed to fall on Bruce at all. Sam ended up liking it and put the scene in the film.