To act in an understated manner or with little expressiveness

simple past tense and past participle of underact

present participle of underact

Third-person singular simple present indicative form of underact

Less than normally active.

plural of underaction

Subordinate action; a minor action incidental or subsidiary to the main story; an episode. / Inefficient action.

insufficient activity

Insufficiently actuated

simple past tense and past participle of underactuate