If you have ever had your 35mm film rewound into the canister accidentally, or need to pull the film lead out again for multiple exposure, then you will need a film retriever.
 
There are a few film retriever available on the market, but we find like this one for its compactness, which means you can easily pop one into a camera pouch. 
 
An instructional video is available here: https://vimeo.com/303310280