An Item card is able to Exert on the turn that they are played.
A Character card is unable to Exert on the turn that they are played. In the Quick Start Guide, this is known as having to "wait for their Ink to dry". A player will be able to Exert their characters only if they were in play since the start of that turn (typically the following turn after they are played).
There are abilities that allow you to Exert a Character. An example of such an Ability would be Elsa - Snow Queen's FREEZE Ability.
Exert chosen opposing character.