Go ahead, make my day!

Eastwood knew exactly what he was doing during his speech. The "befuddled old man" was part of the shtick. It was deliberate. Did anyone notice how his manner and voice changed when he let out his straight political lines?

The speech might not have been tightly scripted, but you can bet that Eastwood pretty much knew before he started what he was going to say and how he was going to say it. And how to ad-lib as needed, and time for laughs.
He's a successful professional actor. I was pretty active in the theater for many years and I saw a lot of good actors at work.