Presumptive
Part of speech: adjective
Definition: based on presumption, probability, conjecture, hypothesis or belief making presumptions; behaving as one who presumes, who assumes that which they perhaps shouldn't.
Believable
Part of speech: adjective
Definition: Capable of being believed; credible.
Example sentence: You can't be funny for funny's sake. You try to get as outrageous situation as you can but it always has to be believable and based in real character motivations and what people would really do.