If you rehearse a prewritten speech enough, you can get asymptotically close to the sort of engagement you get when speaking ad lib. Actors do. But here again there’s a tradeoff between smoothness and ideas. All the time you spend practicing a talk, you could instead spend making it better. Actors don’t face that temptation except in the rare cases where they’ve written the script, but any speaker does. Before I give a talk I can usually be found sitting in a corner somewhere with a copy printed out on paper, trying to rehearse it in my head. But I always end up spending most of the time rewriting it instead. Every talk I give ends up being given from a manuscript full of things crossed out and rewritten. Which of course makes me um even more, because I haven’t had any time at all to practice the new bits.