"You need to find an actor who can find a genuine sense of emotional authenticity and depth, who can do some of the heavier stuff, but can also be really funny and light on his feet and improvise," Webb told Total Film.
"There was a moment when I was like, 'I don't know what we're going to do'. And [Andrew Garfield] came in late in the game and it was a pretty stunning moment."
Webb continued: "And what was fascinating about Andrew is the lines were only a part of it. He behaves in such a way that he communicates the subtext or the text of a scene in such a profound way. You can watch it without the sound and you know what he's feeling."
He went on to discuss the casting of Emma Stone as female lead Gwen Stacy.
"Emma came in and - boy! - she is so on top of it, and she can rat-tat-tat, and she's just fun," Webb recalled. "And there was a lightness to her that was really great."
"And, I remember putting her and Andrew together in a scene and they could just... they were dancing around each other, they were great. It was so much fun to watch these two incredibly intelligent, profoundly talented actors early on in their careers."