Some characters with this fear are worried about becoming something dark and dangerous—a murderer, an abusive parent, or a psychopath. Others might worry about becoming the person they never thought they could or would be when they were young, from a mercenary capitalist to something more mundane, like a stay-at-home mom driving a minivan. This fear, taken to an extreme, could lead a character to not fully explore their own psyche, emotions, personality, and needs, giving them a skewed view of who they are and hindering the development of their true self.

Practicing asceticism or strict religious practices to prevent unwanted behaviors
Avoiding certain groups of people or situations where the undesired behavior may be triggered
Attending long-term therapy
The character seeking reassurance from others that they're not taking on certain traits or attitudes
Going to extremes—e.g., someone who struggles with a forbidden sexual desire deciding to live a life of complete abstinence
Overcompensating
Voicing criticism of people who represent what the character fears
Becoming defensive if someone says something that suggests the character is similar to what they hate
Living a persona (pursuing hobbies, embracing opinions, etc.) that isn't true to who they are
Avoiding interactions with the kind of people the character doesn't want to emulate
Cutting people out of their life who represent what the character hates or who push them in that direction

Feeling guilty over secret desires to be "that person"
The character trying to accept facets of themselves while also staying true to their morals or principles
Struggling to avoid losing control of emotions or actions