Here is the first of 5 checklists I have put together to help during revisions (with some additions in 2014: I'm up to 46 items on this checklist).
A picture of a handsome character doesn't hurt when working on your characterization, either. This one is from Pride and Prejudice (Rupert Friend as Wickham).
1. When you introduce your main character, is she/he doing something - an action distinctive to him/her that also raises a question and sets a hook?
2. Do you clearly state what your main character wants within the first few pages, and what's standing in their way?
3. Does your main character make an active choice instead of just reacting to what's thrown at him/her?
4. How many ways is your main character distinctive from others around him/her? Is it clear right away who the main character is? Does the main character get enough space? (in other words, don't introduce a bunch of characters all at once)
5. Does your main character have at least five problems that need "fixing"? Is at least one of those five things an internal conflict? Does he/she have an old wound from the past that affects them currently?
6. Does your main character have a "save the cat" action in the first five pages? (source: Blake Snyder)
7. Does your main character have some quirk, power, or characteristic we admire and would like to have? (source: Shallee MacArthur)
12. In the opening scenes, does your character state (in a dialogue) their view of the world and their circumstances? Luke Skywalker: "Well, if there’s a bright center to the universe, you’re on the planet that it’s farthest from." (source: Plot to Punctuation)
7. How many ways can you get the reader to "bond" with your character in the first chapter? (source: my post)
8. Does your main character have an affect on other character(s) around him? (source: Donald Maass)
9. Is your main character affected by other character(s) around him? (source: Donald Maass)
10. Does your main character have at least one difference from their stereotype (and possibly more)?
11. Does your main character have a distinctive view of the world? A distinctive attitude? (e.g. rebellious, inquisitive, carefree?) Does he/she directly or indirectly state the motto he/she lives by? (source: Roni Loren)
12. Does your character have distinctive dialogue, specific words they use? (source: Carolyn Mackler)
13. Does your main character purposefully notice somethings and ignore others? (source: Writer's Digest)
14. Does your main character's dominant sense (hearing, vision, taste, smell, touch) come through in the first scenes? (source: Writer's Digest)
15. Does your main character's temperament come through in the first scenes? (competitive, problem-solver, peace-keeper, impulsive, pessimistic, etc - Meyers-Briggs interview questions are a great prep for this)
16. Does your main character have both specific and universal traits? (source: Linda Sue Park)
17. Does your character change over the course of the story? (character arc). Do they realize they need something other than what they thought when they first started out? Is the character arc related to the story's theme? (source: Stan Williams)
18. Have you filtered descriptions through your main character's point of view, for instance how does a particular setting make him/her feel? (source: Story Sensei)
19. Do each of your characters have a goal, something important that they want, and preferably in some sort of conflict with the main character's?
20. Is there at least one thing standing in the way of each character's goal?
21. In the opening scenes, do each of your important characters face a choice or handle a conflict in a way that defines their character? (source)
22. When a new character is introduced, do their interactions with the main character reflect their differences and showcase the side of the character that is most important to the narrative? (source: Sarah Crowe)
23. When a new character is introduced, is he/she introduced in a setting that reflects some aspect of his/her character? (source: Plot to Punctuation)
24. When a new character is introduced, does he/she come with a hint of mystery or something that makes you want to know more about their backstory? (source: Plot to Punctuation)
25. Does your main character have at least three things in opposition to (or opposite qualities of) other important supporting characters? (source: Donald Maass)
26. Is there at least one thing about important supporting character(s) that your main character will never understand? (source: Donald Maass)
27. Do your characters do at least one common thing in an uncommon way? (source: Donald Maass)
28. Do your supporting characters have at least one difference from their stereotype?
29. Are each of your characters hiding at least one thing?
30. Do your characters have distinctive voice and dialogue from each other, preferably reflecting their personality?
31. Do your characters have distinct body language and actions in line with their personality? (source: Patricia Wrede)
32. Is at least one of your supporting characters a foil?- a best friend, sidekick, mentor, romantic lead, parent or virtually any other character who serves as an example or contrast to the main character (Source: Adventures in Children's Publishing)
33. Is at least one of your supporting characters a mentor to the main character, providing help, advice, or support on their way to reaching their goal? (Source: Adventures in Children's Publishing)
The antagonist (if your antagonist is a person and not an event)
34. Does your antagonist have at least one likeable or sympathetic characteristic?
35. Does your antagonist have a valid reason for their resentment (or other motivation)?
36. Does something happen to your antagonist that changes his/her course or intensifies his/her goal? (source: my post)
37. Does your antagonist have at least one difference from their stereotype?
38. Does your antagonist want or need whatever he/she is going after at least as much as the main character wants to achieve his/her goal? (Source: Adventures in Children's Publishing)
2014 addition: some great ideas from James Scott Bell's book, Write Your Novel from the Middle, that I thought should be added to the checklist:
39. How original is your character? Unique in some way?
40. How unpredictable is your character? Does he/she make decisions or respond in ways the reader won't see coming?
41. Does your character feel passionate about something? (goals, beliefs, etc)
42. What is your character opinionated about?
43. Does your character have opposing needs or desires, or experience conflicting emotions?
44. Is your character resourceful? do they solve a problem with a special skill or talent?
45. Do you show your character doing something even though they don't want to do it?
46. Does your character have a noble purpose? Do they overcome their own internal issues and fears or selfishness to act on behalf of others?