All 3 Uses of

mischievous
in
Me Before You

We watched some Japanese animated film that Will said was perfect hangover viewing, and I stuck around—partly because I wanted to keep an eye on his blood pressure and partly, to be honest, because I was being a bit mischievous.†
p. 301.9

Definitions:

(1) (mischievous)
playfully causing minor trouble; or describing the smile of someone doing so

Much less commonly, mischievous can reference real harm without any sense of fun. But in modern writing, that usage has largely shifted to other words like malicious, destructive, or damaging.
(2) (meaning too rare to warrant focus)
meaning too rare to warrant focus:

In law, mischievous references a property crime such as vandalism or graffiti. Very rarely, the word can reference someone or something causing serious damage.

In archaic literature mischievous often refers to bad behavior without any connotation of playfulness or of the harm being minor.