I'm a plotter, meaning that I develop and plan my novels before I ever start writing them. In my case, this is almost something of an extreme, as I develop every part of the novel carefully and then create a very detailed outline scene by scene before writing a word.
I generally have very little trouble with characters. I develop them like everything else, creating them with all the aspects they need. As I develop them, I get a general sense of who they are, and when I sit down to write, I can visualize them acting out what I have them do. This is all I really need to 'get in my character's head.' It works fine.
With my most recent work, I have a bit of a problem. I had originally created the premise for this work several years back. A year or two after that, I partially developed it, including the character, and wrote something of a first draft. I then moved to something else, and didn't come back to it until now, several years later. During re-development, a lot of things changed, including several key aspects of the character and who he was.
The problem is that when I now sit down to write, the character is coming across as an emotionless cardboard stick figure. It is my theory that this is because I have two versions of him in my head, which are essentially complete opposites in some areas (and thus cancel each other out). I think I know the previous version better and might still see him that way as I write, while trying to write him as the new version.
I think I can fix this problem by getting to know my character better. What can I do to get to know my character better?
Important Note: Do not misunderstand this question as 'how can I discover my character?' I already know who my character is and how he acts. What I need is the ability to write him that way.
After-answer-notes: I've marked Lauren's reply as the answer, but if it doesn't work for future viewers, the answer from Mike C. Ford is the way to go. Especially that bit about renaming the character. That is pure genius.