How can I tell someone that I was just making the conversation long for fun? I am looking for some American idiom to express it.
If you are pretending not to understand or misunderstand for a little fun at the expense of the other person that would be Teasing.
If you are just talking to someone with mindless chatter that is often refered to as Babbling.
If you are just talking about things that interest one or both of you then it is often referred to as Chitchat.
If it is a mutual session of playful verbal jabs at each other it can be referred to as Banter
And talking with a member of the preferred sex, for fun is often called Flirting.
I could have been more succinct, but I wanted a prolix conversation for fun.
1 : unduly prolonged or drawn out : too long
2 : marked by or using an excess of words
Huh, good question. If you're looking for a response to a question like "Why is this conversation so long," witty options may include "It's the journey, not the destination," "If not this conversation, then which?" or "Good things come to those who wait." something playful/humorous should render the guy into a more docile state.