“Sean Penn is the guy most commonly associated with Fast Times at Ridgemont High these days, and even though his role is relegated to the B-plot, I can see why his character has endured. He’s great. If I didn’t know anything about the rest of his career, I would have assumed he was a talentless stoner they brought in off the beach and paid in Hawaiian shirts and Fritos. He’s so perfectly oblivious it looks like he showed up on set by accident. It’s a pretty impressive turn to be so broad and also so believable, especially in a movie that tends to be more earnest and deliberate in the rest of its scenes. He’s got the lion’s share of the written jokes, too, but some of the funniest stuff in the film comes from his reactions, obviously an organic byproduct of his total dedication to the role.”

—Alden Ford, “Watching 'Fast Times at Ridgement High' For the First Time”

The rest of the article doesn't do much for me, since, unlike Alden, high school kind of wrecked me. It wasn't a shrug, as it was for him.

But the above is a great encapsulation of Penn as Spicoli. As someone who's had to encapsulate that performance, too, I'm particularly jealous of the “so broad and so believable” riff. That's spot-on.