Harrison Ford created two of the most iconic and enduring characters in movie history, Han Solo and Indiana Jones, but we don't tend to give him a lot of credit for it. They were created by two equally big names, George Lucas and Steven Spielberg, and aren't so much feats of acting as charisma; the fact that the two characters are so similar allows us to think that Ford is basically just playing himself, one time with a spaceship and another time with a whip.
And despite Ford's current reputation for sleepwalking and scowling through whatever comes his way, there was a time when he was a committed and incisive actor, enough to go through a script line by line and ask the kind of difficult questions even the screenwriter himself wouldn't ask. You can see it for yourself in this amazing find, a copy of the original Raiders of the Lost Ark script with Ford's own handwritten notes, asking questions and digging deep into the film's plot. A handful of pages were posted at Cinephile Archive; you can see two of them below and click to see the rest over there.
Aside from his insistence on misspelling Belloq (seriously, he's got the script right there!) Ford makes a lot of really interesting points, like asking if Indy is a believer-- a fair question even for those of us who have seen all four movies. On other pages you can see him altering lines of dialogue for other characters, a pretty bold move, but something I guess you can get away with when you're already pals with Lucas and Spielberg. It's unclear where the full copy of the script is and who owns it, but come on-- this script belongs in a museum!
