From its origins in 1962 to about 1994, Spider-Man was known for having dynamic real-time characterization where characters grew and changed, even after it adopted a sliding time scale. Death was death and so on. The Clone Saga ended its realism, and One More Day exchanged the dynamic characters with Static characters. Likewise most of the story is told from Peter's POV and we rarely see the story from the viewpoint of other characters, so their characterization marched on at a different pace:
Pre-OMD, you had Spider-Man who started off as hotheaded and ready to fight for little reason. As early as Amazing Spider-Man #1, Spidey breaks into the Fantastic Four's house to fight them in order to prove his worth as a potential member. He mellowed out once he became an adult so a lot of this could be chalked up to him being a teenage boy who just got super powers.
Gwen Stacy was easily the most inconsistently written character in the classic period. When co-creator Steve Ditko was around, he consistently wrote her as a stuck-up college-aged Alpha Bitch and beauty queen who got in because of her class and looks. Then after he stepped down, and John Romita wanting a more regular social circle and a change of scenery, had her rewritten into a more virtuous girl. Then her father was introduced and she became a weepy Daddy's Girl who largely followed the men in her life. Most notably she was the Veronica and then the Betty in the ongoing Love Triangle with Peter and MJ, before winding up Spider-Man's very sweet girlfriend, and then, her father died which she blamed on Spider-Man, which did a number on her relationship with Peter who was convinced that she wouldn't accept his double life at all. Then there came The Night Gwen Stacy Died, where posthumously she became a Satellite Character for Mary Jane (right from her death issue, where MJ is easily the most important female character), and MJ often revealed her more vulnerable side whenever Gwen came, feeling upset about her poor background compared to hers and guilty about taking her place in Peter's life. Her later portrayal (Peter's one true love who was Too Good for This Sinful Earth) didn't exist until after her death.
Mary Jane Watson was set up by Aunt May as a Blind Date for her wallflower love-shy nephew (as she saw it) even if Peter felt he was actually doing a good job getting past that (he wasn't but he did have a relationship with Betty Brant). Aunt May kept hyping MJ as an ideal match for Peter and readers, after being told contextually that she was indeed very beautiful, were in suspense for her introduction. And when revealed in #42, as a charismatic Audience Surrogate (a 60s party girl who thought Spider-Man was cool and so was Peter), despite her characterization by Lee and Romita as a flaky party-girl, was seen by fans to be the more interesting character. One of those fans, Gerry Conway, wrote the death of Gwen Stacy specifically to develop her character and revealed a more courageous, compassionate, and loyal side than previously expected. Later writers, Marv Wolfman, Roger Stern, Tom Defalco revealed a much more complex character and made her one of the most important supporting characters not just in Spider-Man but Marvel Comics as a whole, and finally Peter's wife, and Post-OMD still his best known and most popular love interest.
Aunt May is another example. In the beginning, she was a sweet, extremely old and extremely frail old lady but, readers also felt that she was somewhat senile and detached from reality. That she allowed herself to be charmed by Doctor Octopus and saw Mary Jane Watson as a suitable partner and future wife for Peter (before MJ's characterization marched on) was portrayed as evidence for this by writers. Then in the late 1970s she became more involved in the real world, e. g. joining the Gray Panthers, a bypass operation removed her recurrent health problems, and by all appearances she actually became younger. During Roger Stern's run her reasons for continuing matchmaking also was revealed as much more canny than previously imagined; she commented to Peter that both he and MJ "had lost so much" which stunned Peter when he realized that in all the time they dated, he never asked MJ about her life and later he learned that MJ really was someone he had more in common and that his Aunt was right about her all along. Writers also implied that Aunt May knew Peter's secret, which was confirmed in the wonderfully written (but later retconned) issue #400. After the Clone Saga, May discovered the secret a second time in The Amazing Spider-Man (J. Michael Straczynski), has a sane conversation with him about it, and is totally able to deal with it, making her the coolest old lady on Earth. Later episodes have her helping with the secret identity in ways that make you wonder how he ever got along when she didn't know. Then, it was all retconned a second time.
In the Post-OMD era, with static characters for all, in essence characterizations are composite from different periods in history since the undoing of the marriage and the concurrent maturity and growth that comes from characters making that commitment, meant that Peter in Post-OMD is an unlucky but optimistic Butt-Monkey and Manchild, Aunt May remarries, grows younger, and lives apart from Peter but still remains his only family. Mary Jane is Peter's on-off girlfriend, sometimes bitter, sometimes loving and friendly, other times distant, and other times flaky like always.
When the scientist Spencer Smythe - the creator of the Spider-Slayer robots - first appeared, he was a polite, reserved man, who seemed to mostly have created his first Spider-Slayer as he genuinely believed J. Jonah Jameson's editorials about how Spider-Man was a menace. When Spider-Man defeated his first robot, he took it in stride, in a sort of, "Oh well, I guess I could have built a better robot," manner. When he built his next robot, he had become a ranting maniac who took things to a level that was too far for even Jameson.
His son Alistair similarly changed over time - when he first appeared he was what would be known now as a neckbeard, who leapt to wrong conclusions easily and ranted similarly to his father. Over time he became slimmer and turned into more of a sinister, cunning villain.
