A fair concern! That's why Creator/Producer Larry Gelbart didn't tell the cast of M*A*S*H (except Alan Alda) that they were killing off Colonel Blake until just before they acted it, distributing the last page only after everything else had been shot. Hear him tell it: https://www.youtube.com/watch?v=g3Rh2EkQWhw .
(Oh, yeah, spoiler alert... for a show that aired 43 years ago. )
The counterargument: A performer who knows what's coming up can temper their pacing -- put the high and low points in the right places. I'm glad I read my Monte Cristo part ahead of time, but probably I'll try it the other way eventually.