Present participle of costar; currently appearing as a costar, or to appear together as equal main actors.
Present participle of costar formed with -ing suffix, following English grammar patterns for continuous/progressive forms of verbs.
The fact that we have 'costarring' as a continuous form shows how 'costar' became a real verb—it wasn't just invented, it integrated fully into English grammar, getting all the verb forms and conjugations.
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