The theme of parent-teen relationships is strong in Liar, Liar as we see the troubled relationship between Sean and his father. Sean yearns for a parent to talk to who will listen without being judgmental. Dad always seems occupied with his work and Mom is too far away. Not knowing how to relate to Sean, Dad frequently uses gifts and the promise of outings together to smooth things over, when all Sean wants is to talk. This is a common problem between parents and their teen-age children and helps to make the novel meaningful to young readers.

Mortality confronts the reader in Liar, Liar as Sean thinks about real life following Marsh's death. "I mean, once Marsh and I thought nothing would ever happen to us. No matter how bad the accident, we would walk away from it just like Burt Reynolds always does...