I know there can be some confusion about adapting work for the screen and owning the rights.
If you are absolutely in love with a novel, cartoon, graphic novel (or ANY work that is not your own) and feel that it is your life’s destiny to adapt this work for the screen, then GET THE RIGHTS before you begin.
It doesn’t serve you in any way to write a script that is based on material you don’t own the rights to.
It is possible to acquire the rights to adapt something. You will likely have to contact the original author, or their representation, and negotiate an option (a term of rights) on the material.
I recently read a script wherein the writer introduced his main characters as being “exactly like” other very famous cartoon characters and mentioned the cartoon by name. Aside from telling me these characters were really someone else’s characters, the author provided no other introduction to his characters. PASS.
I personally would strongly discourage an independent, unestablished writer from optioning and adapting material, simply because you will invest a large amount of time actually writing the project and then you’ll be left with not too much time to shop it. If anything were to come of that project before the option expires, you would be lucky.
Meanwhile, if the source material has become more popular within that timeframe and the author gets a better deal on another option, your option will expire and the project will move forward without you. It is unlikely anyone will ever read your draft and then you will have basically wasted all of those months – when you could have just been working on your own original material and then would own it outright.
One of my screenwriting teachers in film school had a mantra: You can’t copyright an idea. Ideas are a dime a dozen. The only thing you can protect is your specific vision of that idea. To that end, if there is something you are very passionate about, find a way to reconceive the material that is unique to you and your thematic and vision and write the hell out of it. Even if it is a direct nod to another preexisting project, people will likely get that but then see your original voice in it – which is a lot better than just being a hack.