Assume isn't only used to mean "accept as truth without checking"; it also means "take on the form of." It might be safer if you don't assume that the vampire standing in front of you isn't merely a person assuming that form.
Assume always has the sense of taking on something. It may be the belief in the appearance of truth: Your mother probably assumes you do your homework right after school. It may be another form or identity: Superman assumes the identity of a city reporter. Or, it might be a physical space: If you get nervous while driving, your dad might assume control of the car.