Plural of costar (actors who share main roles), or third-person singular present tense of the verb costar.
Either plural noun form or present tense verb form of costar, both using standard English grammatical formations.
The same word 'costars' can be a noun (multiple actors) or a verb (he/she/it costars), which happens often in English—words get recycled for different grammatical jobs, making our vocabulary incredibly efficient.
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