Personally, a couple of my favorite Eastwood lines come from Heartbreak Ridge, where Sergeant Major Choozu and Gunny Highway are exchanging insults when the reporting-in Highway is caught drinking his old buddy Chooz's own coffee from his own cup:

Chooz: Hey there, skunk-stool! What are you don', sucking down my lifer's juice out of my own mug?

Highway: You're right, I should've had my shots first. But I figured I'd take a chance.

Chooz: If your brain ran as fast as your mouth, you'd be a friggin' 20 star general by now.

Highway: And you, Sergeant Major, ought to be a poster child for a prophylactic.

Only two old friends could get away with slanging each other that way.