Question: Say I get a patch created with `git format-patch`. The file is basically a unified diff with some metadata. If I open the file in Vim, I can see which lines have been modified, but I cannot see which *characters* in the changed lines differ. Does anyone know a way (in Vim, or some other free software that runs on Ubuntu) to visualize per-character differences?

A counter example where per-character diff is visualized is when executing `vimdiff a b`.

**update Fri Nov 12 22:36:23 UTC 2010**

[diffpatch is helpful](https://stackoverflow.com/questions/242646/use-vimdiff-with-a-diff-file/244766#244766) for the scenario where you're working with a single file.

**update Thu Jun 16 17:56:10 UTC 2016**

Check out [diff-highlight in git 2.9](https://github.com/blog/2188-git-2-9-has-been-released). This script does *exactly* what I was originally seeking.

Answer: 
In git, you can merge without committing. Merge your patch first, then do:

```
git diff --word-diff-regex=.

```

Note the dot after the equals sign.
Given your references to Vim in the question, I'm not sure if this is the answer you want :) but Emacs can do this. Open the file containing the diff, make sure that you're in `diff-mode` (if the file is named `foo.diff` or `foo.patch` this happens automatically; otherwise type `M-x` `diff-mode` `RET`), go to the hunk you are interested in and hit `C-c C-b` for `refine-hunk`. Or step through the file one hunk at a time with `M-n`; that will do the refining automatically.