If you hit pause anytime a train goes by, because all the animators wanted to animate Stan Lee, he's in almost every single train.
It was Peter Ramsey's idea to hold off on the visual comic language - word bubbles, panels, etc. - until Miles is bitten by the spider.
It was announced shortly after Stan Lee's death, at age 95, that he had recorded a cameo for the film and that it would be his final voice-acting role. Phil Lord and Christopher Miller felt it was important that Lee was given a bigger moment compared to previous Marvel films because he was "so integral to the spirit of this movie," and considered his role "extra meaningful" following his death.
Art directors Dean Gordon and Patrick O'Keefe and their team turned to Cubism to help represent the dimensional quakes. Cubist art often presents a collection of different views all happening at the same time, so it was a natural metaphor for the multiple universes converging in "Spider-Verse."
(at around 52 mins) The "bagel" text as the bagel hits the scientist's head was a joke pitch that an animator took seriously and added in. They loved it.