Trim the fat. Break the novel down to its basics.
Novels are kind of like a juicy roast pig with lots of side dishes. The central quest is the main course, but you also have many delicious side quests, filler chapters, B-plots, C-plots, etc. You have time to develop many different details of the worldbuilding.
Comics and movies are like lean meat, especially movies which are probably only about two hours long. There's not enough time for countless side quests and B-plots. You need to be laser-focused on the singular goal.
So, if you are taking a bulky novel and adapting it into a different medium, you need to cut out the fat and break the narrative down to its most basic form.
A good example of this would be the Lord of the Rings adaptation. In the books there was a time where the hobbits spent some time in the Old Forest where they met a character named Tom Bombadil and his lovely wife, who offered the group shelter on their journey.
Why was this encounter cut? Time. In the books, it was a fun side quest to expand on the world a bit. Keeping it in the movies would have been pointless because it doesn't contribute anything to the main adventure.
When cutting dialogue in an adaptation, consider the following:
-Is this contributing to the main plot or is it a sidequest?
Even if the scene is fun, it might need to be cut if it wastes time and does not contribute much to the plot.
-Which scenes are the most integral to the character and their journey?
Don't cut scenes that are important to developing the characters, or they'll end up feeling hollow.
Despite being good adaptations overall, the Harry Potter movies tended to do exactly this, eliminating scenes that would have really helped flesh out the characters. One of the best examples was in Order of the Phoenix where they left out a scene for the character Neville.
His parents were driven insane by Bellatrix Lestrange, a loyal follower of Voldemort, but the movie cut out the sad scene where we get to see the state of disrepair his parents are in. It changes Neville from a clumsy side character to someone with an incredibly tragic past.
Cutting it eliminated a lot of Neville's character depth and makes his triumphs at the end feel a little weaker if you only know him from the movies.