How does one run it?

Using Avast! 2014 Premier - if I have got a file which I know is infected and want to run it in the Sandbox without it getting quarantined before I can see what it does i.e. what files it drops etc. - how do I do it?

Right click the file in question and select run in sandbox

Thats exactly what I do - the virus detection window slides up indicating that a virus has been detected and that the file has been moved to the chest, and thats it, nothing more, nothing less.
If you selet the scan with Avast option on the file it also detects the virus (that how I know its infected) and then you have the options to decide what to do with it. Sandbox, no options, straight to the chest. My understanding of the sandbox is that it would let you run the file in isolation and then log what it does, which is what I am interested in.

Avast will not allow that, if it knows the file is infected it will kill it unless you run it inside a virtual machine with no antivirus

Pardon the ignorance - but why would you then need a sandbox, the active shields will be doing just that when you try to run the file, won’t they?

The sandbox is used to test a file you are unsure about in a controlled environment
If it does turn out to be bad then Avast will terminate it and add it to its list of bad boys

Sandbox is not designed to run malware files to see what they do for that you need a VM

you can upload the file to ThreathExpert. http://threatexpert.com
or Anubis. http://anubis.iseclab.org/?action=home