Just a few weeks ago I was in a group of other writers and someone asked, "How do I make a strong antagonist?" The thing to keep in mind is, the antagonist doesn't necessarily have to be evil. The antagonist is someone who prevents the protagonist from reaching their goal. It's important to note that, like your protagonist, your antagonist needs to have some kind of goal, but somehow ultimately lose in the end. Just like how your protagonist has a background or origin story, your antagonist should also have some kind of background as well so we get a sense of who they are. For example, in Joker, we all know Joker is a well known villain in the Batman franchise, but in the film Joker is the protagonist. The main theme of the film is man vs society, so as the film goes on we understand where he's coming from.
When you're making your antagonist, it's important to note that they should be someone we shouldn't hate at all. For example, let's take a look at Back to the Future. Yes Biff came off as a jerk, but as a viewer, I wouldn't say I hated him. Sometimes there can be an antagonist that can make you feel so upset you completely lose interest in the film overall.
In Mean Girls, Regina George is your typical high school antagonist, but as the film goes on, we understand that although she's a plastic on the outside, she has a heart of gold on the inside and we get a better understanding of who she is. So in a way, she changes in the end and everyone gets their happy ending.
Another example of where antagonist changes in the end would be Mr Banks from Mary Poppins. I've seen this film countless times and each time I watch it, I kept thinking Mr. Banks was the protagonist because he's the character who develops throughout the film. The thing though is, he really was the antagonist and needed to be saved by Mary Poppins herself. To quote Walt Disney (played by Tom Hanks) in the movie Saving Mr Banks, "Mary doesn't come to save the children. She comes to save the father."
You're probably wondering if your antagonist has to change in the film just like the protagonist, and in most cases, I'd say no because in some cases it's rare for an antagonist to develop and change who they are. Let's take a look at Gaston from Beauty and the Beast. Although he's beautiful on the outside, he has the heart of a pig on the inside. His goal in the film is to of course marry Belle, but she has no interest in him. He takes action to getting Belle to marry him when Maurice says that she's at the Beast's castle. At first the villagers in the bar laugh it off thinking he's crazy, but Gaston later decides to blackmail Belle into marrying him by convincing an asylum doctor to take Maurice away. So in the end, Gaston doesn't really change at all.
Sometimes when someone thinks of the word "antagonist" the first word that pops in their mind is "evil". Like I've mentioned, not every antagonist has to be evil. But why are they evil? What makes the become evil? What are their intentions? What is their goal? How do they go about it? Usually in movies, the antagonist spends most of their screen time going through their plan and raising the stakes for the protagonist, but doesn't ultimately fail until sometime in either the end of act 2 or start of act 3.
If you have any questions about this, please feel free to reach out! I'm more than happy to help!