For one off talks, or decks that one doesn't use often, yes you're 100% right. When I'm in that situation I rehearse the talk over and over and over again to the point where I no longer need notes. My goal is to get to a point where I neither sound like I'm reading from a script, nor would I get thrown off if I get interrupted.
However for material that I've presented a bunch of times, I no longer need to rehearse. There was a period of my life where I could on the fly give a talk on a particular topic at a moment's notice. Ideally I'd have a bit of time to customize some bits to the specific audience/venue, and sure I'd rehearse those. But the bulk of it I could stitch together content in just about any order on the fly.
But if you tell me give some variation on your talk about $X, I can probably be ready in about 30 minutes. (And, in fact, I've dropped in a conference presentation in response to a 3am email because another speaker forgot they were supposed to be there.)
All of those previous presentations basically count as rehearsals. Depends on you, but if it's been a year or several since you gave one, it's probably a good idea to do at least a mini rehearsal before you do it again, but if you're doing one a quarter, once you're good, you should be good.
Intros and ends though I agree require careful thought to appeal to the right audience and give the right message.
I don't think this follows. There are many things I am an expert in that I'm not an expert in communicating about impromptu- I've been to many talks where the speaker is clearly an expert but was not prepared to speak.
I certainly think there are people for who expertise == ability to speak about it, but that's certainly not always or usually the case.
I remember the first time I went in front of a live audience to give a 2 minute intro. It was a meetup with ~60 people and all I had to do was go up and talk about a course I was selling.
On the train ride in, I scripted it out on paper and then re-read it like 30 times.
By the time I got off the train and closed the notebook I forgot everything and dreaded the walk to the venue. I got in front of everyone and tried to recall what I wrote, had a quick internal dialog with myself for about 3 seconds and mentally noted "you're a moron, just wing it". Then I winged it.
In the end it converted ~15% of the room on a tech topic that was ancillary to the main meetup's subject.
On that day I learned I can do well writing something out on paper but I can't remember shit when trying to deliver it live. I have to fall into category (A) and trust myself to deliver but really committing to that with no backup plan does make every experience interesting to say the least.
I've recorded around 300 videos since then and I still need to do the intro about 20 times before I get into the flow to do the rest of the video in basically 1 take. They are usually unscripted with no preparation, I just pick a topic and go.
Fascinating. What I learned from your anecdote is that massed practice didn't work, just as the literature on human memory predicts. Spaced repetition still sounds like a winner. Practice, sleep, forget almost everything, then repeat until it's go time.
I don't know why people don't seem to understand that.
If it's a topic that is low stakes and I know a shit load about, I'll practice for a day or two. If it's a topic that I know and it's high stakes, I may practice daily for two or three weeks.
Everyone who knows me professionally remarks about how relaxed I seem when public speaking.
That's because it's all intentional. Every pause. Every single thing. I seem relaxed, because I am relaxed.
In general, the best speakers are those who a) know the material inside and out, and b) have practiced shit loads.