I sometimes have a need to compare a specific file between the trunk of my SVN tree, and another branch of the same code using IntelliJ's built in difference viewer. But I always struggle to set it up correctly. Here is the rundown:

My tree is structured like:

Base -> Client -> Project -> trunk -> src
                          -> branches -> branch1 -> src
                                      -> branch2 -> src
                                      -> branch3 -> src

to compare a file, say src -> com -> package -> File.java between trunk and branch2, you would do this:

1. In IntelliJ, open up the project in trunk
2. Locate file File.java
3. Right click on the file name tab, and select Subversion -> Compare with branch...
4. Click Configure branches

5. Under trunk location, specify the path as:

   Base -> Client -> Project -> trunk
   

6. Click Add... and locate the branch branch2, once selected it will appear in the branch locations list as:

   Base -> Client -> Project -> branches    
   

7. Click Ok.

8. Again right click on the file name tab, and select Subversion -> Compare with branch...
9. Select branches... then in the second popup select branch2.
10. The files will open side by side.