You could probably give your readers a little bit of an upset if you used ‘interred’ when you meant ‘interned’. Unless they really don’t like the character you are writing about – they might just be disappointed when they realise the mistake.

• Interned: imprisoned (particularly for political or military reasons). This also means you should use internment in this context.
• Interred: buried (typically with funeral rites). And this should be interment.

These are words to keep in mind if you tend to accidentally hit the wrong keys as you type – the spellchecker won’t object to them and the difference (n/r) is easy to skim over when you check your writing.