THAT look.

The look Obi-wan immediately gives Luke after Luke asks him "how did my father die?". It's a look that embodies that question, it says it all. It's like Kenobi always expected to hear that question one day.

Just look at his eyes, it is a masterclass of acting from the now sadly gone Alec Guinness. Say what you like about George Lucas but the sheer fact that he was able to interest Guinness (who was already an acting legend in '77) in a role like this, well, I say all credit to him. The rest is history.