How do I generate a response file to be used with apt-get or aptitude?

I need to install exim4 through puppet. Because this installation requires interactive configuration, I need a response file, but I don't know how to generate one. So, how do I generate a response file?

Use debconf-set-selections to pre-populate the configuration database before you run apt-get. You can use debconf-get-selections (in debconf-utils) on a configured system to see what needs to be set. Also keep in mind that with exim, the package you actually need to configure is exim4-config.

debconf-get-selections | grep exim4-config
exim4-config    exim4/dc_noalias_regenerate boolean false
exim4-config    exim4/dc_smarthost  string  
...

See also http://wiki.debian.org/DebianInstaller/Preseed