un·der·act 
(ŭn′dər-ăkt)
(ŭn′dər-ăkt)v. un·der·act·ed, un·der·act·ing, un·der·acts
v.tr.
To perform (a theatrical role) weakly, with insufficient expressiveness, or with intentionally understated expressiveness.
v.intr.
To perform in an understated or unemotional way.