There are a few different ways to unlock secret characters in Crossy Road. First, many characters can be unlocked by playing the game and are randomly obtained when you reach certain milestones.
Some characters require you to complete specific tasks, such as jumping on certain objects or reaching a certain distance in the game. Also, keep an eye out for seasonal events, as some characters are only available during special event periods.
To maximize your chances of unlocking them in crossy road, you should spend your coins wisely on mystery boxes and participating in events to collect more characters.