Seems like an oxymoron, doesn't it?  "Prepared spontaneity"?  How can you prepare for spontaneity?

Well, that's what we do for most of our behaviors.  For example,

--Driving a car.  You don't want to have to think before hitting the brakes.

--Playing most sports.  Tennis?  In a fast game, you have to hit your strokes with "unconscious competence" without thinking in advance.  That's what Tim Galwey wrote about in his classic "The Inner Game of Tennis."

--Playing musical instruments, from accordions to zithers, requires an automatic deftness that come from preparation and practice.

--Even Japanese "Tea Ceremony" requires an artistry that comes only with practice.  Only then can the Tea Master perform the exact moves without thinking.

--Interviewing for a job?  Rehearse with a coach until you can respond to tough questions briefly and fluently.  Looking awkward and hesitant won't get the job.

--Since Dwight Eisenhower, presidential candidates have rehearsed for TV debates with coaches and staff members .

When I used to act in plays, it was my purpose to learn my scripted words until I owned them and became the character speaking them exactly so I wouldn't sound rehearsed!  Whether Shakespeare, Chekov, or Tennessee Williams, we actors had to own and inhabit the words.  

Before I present speeches and seminars, I also rehearse my materials  in order to know how they "feel" and what they sound like when spoken.  

When I played with a comedy improv troupe, we held weekly workshops to practice trusting our ability to respond when we got an "offer" from the audience or from another player.  "Tell a story about when Donald Trump went deep-sea fishing."  The best players develop the ability to intuitively come up with appropriate responses because, like the best athletes and dancers, they are in "the zone."  As a professional dancer once told me after her performance:  "I wasn't dancing the dance.  The dance was dancing me."  

For fluent conversation in everyday life, there is no need for you to memorize a script.  However, you'll find it helpful to rehearse certain "moves," such as how to transition to a different topic and how to tactfully end a conversation.  Having such moves will give you confidence, and, being confident, you'll converse with ease.

My late colleague, Professor Jason Alter, was an international expert in teaching English as a second language.  One of his most effective class exercises was "sponting," a lively activity in which students responded quickly without thinking or going through a mental translation process.  His goal was that the students move from self-consciousness to self-confidence so they could trust their ability to speak in the moment.  Many of his Asian students eventually went on to top universities in the U.S. and became diplomats and international business people.   

Rehearsing your conversation "steps" will allow your words to "talk you."