Differences between play and movie

I don't think there were any significant changes, In fact it was very faithful, and at times word for word; however, I did notice one minor change.

At the beginning of the play's second act(i.e. the next morning)Willy instructs Biff on how to behave at his job interview. One of the things he mentions is that if something falls off the desk, don't pick it up; because that's the office boy's job. Later when Willy visits Howard, something falls off and Willy picks it up. This was meant as another example of Willy's hypocritical nature(the first example was when Willy said that the Chevrolet was the greatest car ever made, and then, when Linda says that it takes up alot of their bill, he says that they should stop manufacturing Chevrolets)

In the movie Willy instructs Biff, but nothing falls off the desk.

Other than that there's practically no difference.

In the script of the play, Willy attributes his lack of success to being "fat" and says somebody called him a "walrus." Since Dustin Hoffman, who is not really extremely tall, is playing the role, those lines are changed to "short" and "shrimp." This production reinforces Willy's "Loman/low man" status by having practically every actor in the supporting cast, even Kate Reid and Charles Durning, be taller than Willy (if only slightly, in those cases). This is really used to effect in a shot where Biff and Happy flank, and tower over, Willy, and in Willy's scene with Howard.

Now, please don't use my comments here without proper attribution if you're doing homework of some sort. If you do that, your luck will immediately turn awful, and you will be visited by 100 traveling salesmen within 48 hours.